CHAPTER IV.

THE LACCOLITE.

The principal facts in regard to the laccolites of the Henry Mountains having been set forth in the preceding chapter, an attempt will now be made to deduce from them the natural history of the Laccolite.

There is a question which the critical geologist will be likely to propound, and which should be answered at the outset. “What evidence”, he may demand, “is there that the origin of the laccolite was subsequent to the formation of the inclosing strata rather than contemporaneous with it? May it not have been buried instead of intruded? May not the successive sheets and masses of trachyte have been spread or heaped by eruption upon the bottoms of Mesozoic seas, and successively covered by the accumulating sediments?” The answer is not difficult.

1st. No fragment of the trachyte has been discovered in the associated strata. The constitution of the several members of the Mesozoic system in the Henry Mountain region does not differ from the general constitution of the same members elsewhere. This evidence is of a negative character, but if there were no other it would be sufficient. For there is indubitable proof that at the end of the Shinarump period the Henry Mountain region was lifted above the ocean, and if any of the nuclei of the mountains had then existed they could not have escaped erosion by shore waves, and must have modified the contemporaneous deposits.

2d. The trachyte is in no case vesicular, and in no case fragmental. If it had been extruded on dry land or in shallow water, where the pressure upon it was not sufficient to prevent the dilatation of its gases, it would have been more or less inflated, after the manner of recent lavas. If it had issued at the bottom of an ocean, the rapidity of cooling would have cracked the surface of the flow while the interior was yet molten and in motion, and breccias of trachyte _débris_ would have resulted. The absence of inflation and of brecciation are of course of the nature of negative evidence, but they derive weight from the fact that a great number of distinct bodies of trachyte have been examined in the course of the investigation.

3d. The inclination of the arched strata proves that they have been disturbed. If the laccolites were formed in each case before the sediments which cover them, the strata must have been deposited with substantially the dips which they now possess. This is incredible. The steepest declivity of earth-slopes upon the land is 34° from the horizontal, and they have not been found to equal this under water. Prof. J. D. Whitney noted 23° as the original slope of a deposit on the Pacific Coast, and regarded it as an extreme case. The writer has made many measurements of the inclination of oblique lamination in massive sandstones, and found the maximum to be 24°. But the strata which cover the laccolites dip in many places 45° to 60°, and in the revetments of the south base of Mount Hillers they attain 80°.

4th. It occasionally happens that a sheet, which for a certain distance has continued between two strata, breaks through one of them and strikes across the bedding to some new horizon, resuming its course between other strata. Every such sheet is unquestionably _subsequent_ to the bedding.

5th. The strata which overlie as well as those which underlie laccolites and sheets, are metamorphosed in the vicinity of the trachyte, and the greatest alteration is found in the strata which are in direct contact with it. The alteration of superior strata has the same character as the alteration of inferior. This could never be the case if the trap masses were contemporaneous with the sediments; the strata on which they were imposed would be subjected to the heat of the lava, but the superior strata would accumulate after the heat had been dissipated. In the Henry Mountains a large number of observations were made of the phenomena at and near the contacts of sedimentary and igneous rocks, and in every instance some alteration was found.

In fine, all the phenomena of the mountains are phenomena of intrusion. There is no evidence whatever of extrusion. It is not indeed inconceivable that during the period in which the subterranean chambers were opened and filled, a portion of the lava found its way to the top of the earth’s crust and there built mountains of eruption; but if such ever existed, they have been obliterated.

The Henry Mountains are similar among themselves in constitution. They all exhibit dome-like uplifts; they all contain intrusive rocks; and their intrusive rocks are all of one lithologic type. They are moreover quite by themselves; the surrounding country is dissimilar in structure, and there is no gradation nor mingling of character. Thus similar and thus isolated it is natural to regard the mountains as closely related in origin, to refer their trachytes to a common source, and to look for homology in all their parts. It was the search for such homology which led to the hypothesis that the laccolite is the dominant element of their structure. It is now time to examine this hypothesis, the truth of which has been assumed in the preceding pages, and see how far it accords with the facts of observation.

The facts to be correlated are the following:

1st. There are seven laccolites which lie so far above the local plane of erosion that they are specially exposed to denuding agents. They have no enveloping strata, and their only associated sheets lie in the strata under them. Their original forms have been impaired or destroyed by erosion. They are the Scrope, the Jukes, the Sentinel and its three companions, and the A laccolites.

2d. There are two laccolites so nearly bared that their forms are unmistakable, but which are still partially covered by arching strata, and which have associated sheets and dikes. They are the Marvine and the Steward laccolites.

3d. There are five supposed laccolites situated where the erosion planes are inclined, which run under the slopes and are covered at one side or end, and at the other project so far above them as to have lost something by erosion. These are accompanied by overarching strata and by sheets and dikes. They are the Peale, the Howell, the Shoulder, the D, and the E.

4th. There are seven or eight supposed laccolites, of which only a small part is in each case visible, but which are outlined in form by domes of overarching strata. Their bases are not exposed. Associated with them are dikes and sheets. They are the Hillers, the Pennell, the Geikie, the Newberry, the Bowl Creek, the C, the H, and perhaps the F.

5th. There are five domes of strata accompanied by dikes and (with one exception) by sheets, but showing no laccolite. They are the Ellsworth, the Greater Holmes, the Crescent, the Jerry, and the B arches.

6th. There are nine or more domes of strata with no visible accompaniment of trachyte. They are the Lesser Holmes, the Pulpit, the G, the Dana, and the Maze arches, and those of the foundation of Mount Pennell and of the west base of Mount Ellen.

Upon the hypothesis that all these phenomena are examples of the laccolitic structure, they have the following explanation: Each individual case comprised originally a laccolite, covered by a great depth of uplifted and arching strata, and accompanied by dikes and sheets which penetrated the strata to a limited distance. Lying at different depths from the surface, they have borne and still bear different relations to the progressive degradation of the country, and have been developed by erosion in different degrees. In nine of the instances cited the arch of strata has been truncated, but at so high a level that no dikes nor sheets were unearthed. In five instances the plane of truncation was so low that dikes and sheets were brought to light, but not the main body of trachyte. The truncation was in most of these cases less perfect because of the resistance to erosion by the hard dikes and sheets and by the strata which their heat had hardened. In eight other instances the erosion has left prominent the dome of hardened strata with its sheets and dikes, but has somewhere broken through it so as to reveal a massive core of trachyte. In five instances one side of the dome of strata has been washed away, exposing the core of trachyte to its base and showing undisturbed strata beneath it. In two instances the soft matrix has been so far washed away from the laccolite as to expose its form fully; and seven laccolites have not only lost all cover but have themselves been partially demolished.

Certainly, the hypothesis accords with all the facts that have been observed and unites them into a consistent whole. It explains fourteen dome-like arches of sedimentary rock which are imperfectly exposed, by classing them with seven other arches of the same region which have been opened in section _to the base_ and found to contain laccolites; and it strengthens the case by pointing to a connected series of intermediate phenomena. Until some strata-dome of the Henry Mountains, or of a closely allied mountain, shall be found to display some different internal structure, it will be safe to regard the whole phenomena of the group as laccolitic.

_Form of laccolites._—As a rule laccolites are compact in form. The base, which in eleven localities was seen in section, was found flat, except where it copied the curvature of some inferior arch. Wherever the ground plan could be observed it was found to be a short oval, the ratio of the two diameters not exceeding that of three to two. Where the profile could be observed it was usually found to be a simple curve, convex upward, but in a few cases and especially in that of the Marvine laccolite the upper surface undulates. The height is never more than one-third of the width, but is frequently much less, and the average ratio of all the measurements I am able to combine is one to seven.

The ground plan approximates a circle, and the type form is probably a solid of revolution—such as the half of an oblate spheroid.

_Internal Structure._—Of the laccolites which are best exhibited in section, there are a number which appear to be built up of distinct layers. The Peale exhibits three layers with uneven partings of shale. The Sentinel shows two without visible interval. The Howell shows two. The Pennell has a banded appearance but was not closely examined. The Marvine shows at a distance a faint banding, which near by eludes the eye. No division nor horizontal structure was seen in the Hillers, Jukes, Scrope, or Steward laccolites, but observation was not sufficiently thorough to satisfy me of its absence. It is probable that all the larger laccolites are composite, having been built up by the accession of a number of distinct intrusions.

There is little or no prismatic structure in the trachytes. It is sometimes simulated by a vertical cleavage induced in sheets and laccolites which are undergoing disintegration by sapping, but I did not observe the peculiar prismatic cleavage which is produced by rapid cooling in dikes, sheets, and _coulées_ of basalt. The two structures are not often discriminated, but they are really quite distinct, and their peculiar characters are easily recognized. The cleavage planes produced by cooling are as a rule perpendicular to the cooling surface, and the systems of prisms which are based on the opposite walls of a dike or sheet, do not correspond with each other, and do not run across, but meet midway in a confused manner. The cleavage planes which are produced by the shearing force when a massive bed of trap or other rock is undermined or sapped and yields under its own weight, extend from base to top, and are perpendicular to the plane of the horizon instead of the plane of the bed.[2]

Footnote 2:

See page 172 of the “Exploration of the Colorado River”, by J. W. Powell.

_Vertical Distribution._—The range of altitudes at which laccolites have been formed is not less than 4,500 feet, and neither the upper nor the lower limit is known.

The highest that are known—those which were intruded at the highest geological horizon—are near the base of the Blue Gate shale (Middle Cretaceous), but it is quite possible that higher ones have been obliterated by erosion. The lowest that is known was intruded in the Shinarump shale, but it is known of the invisible Ellsworth laccolite that upper Carboniferous strata lie above it, so that its horizon of intrusion must be still lower. The plexus of dikes and sheets on the Ellsworth arch indicates that the laccolite is not deeply buried; but in the series of arches there are nine which show no trachyte, and we have no data from which to infer their depth.

How the laccolites are distributed within these limits is more readily comprehended by the aid of a diagram. In Figure 50 the triangles mark the horizons of determined laccolites, and the crosses the horizons which the invisible laccolites cannot exceed. For example, the base of the Scrope laccolite is visible and is seen to lie on the Tununk shale 400 feet above the Henry’s Fork conglomerate; to represent it a triangle is placed at about the middle of the space representing the Tununk shale, and in the column devoted to the Ellen cluster. The upper surface of the Geikie laccolite is visible and upon it rest, first a few feet of Flaming Gorge shale, and then the Henry’s Fork conglomerate; to represent it a triangle is placed near the top of the Flaming Gorge space. The crown of the Pulpit arch has been so far eroded that half of the Vermilion Cliff sandstone is shown in section, but no laccolite is revealed. It is evident that the Pulpit laccolite is lower than the middle of that sandstone; and to represent it a cross is placed below the middle of the Vermilion Cliff space and in the column devoted to the Hillers cluster.

[Illustration:

FIG. 50.—Diagram of the Vertical Distribution of the Laccolites of the Henry Mountains. ]

The first feature which this graphic assemblage yields to the eye is that there are at least two zones of laccolites. The upper ranges from the lower part of the Blue Gate Group, through the Tununk and Henry’s Fork, to the upper part of the Flaming Gorge Group. The lower has not yet been fully developed by erosion, but its proximity is indicated. The Hillers laccolite is uncovered at top, and the dikes of the Ellsworth, the Greater Holmes, and the Crescent have been reached. All of the invisible laccolites indicated in the Vermilion Cliff and Shinarump spaces must be referred to this zone; and there is reason to suspect that all but one of the invisible laccolites whose indication falls in the Tununk and Flaming Gorge spaces belong also to the lower zone. However this may be, it is not probable that the determination of the depths of the invisible laccolites would vitiate the conclusion that there is an upper zone of laccolitic frequency which is separated from a lower zone by an interspace of laccolitic infrequency.

Another feature illustrated by the diagram is that all the determined laccolites are inclosed by soft beds. They have been intruded into the shales, but not the sandstones. They cluster about the Henry’s Fork conglomerate, but none of them divide it. This selection of matrix is confined however to the laccolites and is not exercised by sheets and dikes. Trachyte sheets were seen within the Henry’s Fork, the Gray Cliff, the Vermilion Cliff, and the Aubrey sandstones.

A third feature of the diagram is the restriction of the upper zone of laccolites to the northward clusters; Mounts Ellsworth and Holmes contain laccolites of the lower zone only. This fact of distribution is correlated with a fact of denudation—namely, that in the general degradation of the country, the region about the southern mountains has lost two thousand feet more than the northern. One fact is probably the cause of the other. The absence of laccolites of the upper zone at the south may have permitted the greater degradation; or the greater degradation may have caused the destruction of several of the upper laccolites. The fact that the difference in degradation can be independently accounted for is favorable to the latter supposition. The Colorado River which is the main artery of drainage for the whole region, flows close to the bases of the southern mountains, and the rapid declivity from the mountain summits to the river has given and still gives exceptionally great power to the agents of erosion. No other cause is needed to explain the difference of degradation, and the absence of laccolites of the upper zone is explicable without assuming that they were never present.

The negative objection to the idea that the southern mountains originally possessed laccolites of the upper zone being thus disposed of, it is worth while to inquire whether there is any evidence in its favor. If superior laccolites existed, they would be sure to leave behind them a record of the conduits through which their lava was injected. A dike or a chimney must always connect a laccolite with the source of its material; and the removal of the laccolite necessarily exposes a cross-section of its stem. The discovery of such a dike can be regarded, not indeed as a proof of the former existence of a superior laccolite, but as demonstrating its possibility. The summits and flanks of Mounts Holmes and Ellsworth bear many dikes, which have been regarded as subsidiary features of the laccolites beneath them, but it is quite possible that any one of them formerly led to another laccolite above. The upper laccolite may have been first formed and then have been lifted, dike and all, when the lower was intruded; or it may have been last formed, and been fed through a fissure which traversed the lower after its congelation. The only dike which was discovered in the vicinity of these mountains, without being upon them, stands midway between them. It is the only observed dike of the Henry Mountains (excepting always the alpine district of Mount Ellen), which is not so closely associated with some laccolite as to seem an accessory feature, and its exceptional position has led to the suspicion that it belonged to an overlying laccolite.

Upon such uncertain evidence no positive conclusion can be based, and it is vain to build laccolites in the air. The most that can be said is that the southern mountains need not be distinguished from the northern, because at the present stage of degradation they contain laccolites of the lower zone only.

_The Material of the Laccolites._

The intrusive rocks of the Henry Mountains were sampled with care. Specimens were selected which had undergone little decomposition and which represented all the prominent lithologic varieties. They were chosen from the trachytes of both zones and of each of the mountain masses; and they represent dikes and sheets, as well as laccolites. From about thirty specimens thin slices were cut for microscopic examination.

Captain C. E. Dutton, of Omaha, Nebraska, was so kind as to undertake the study of the collection, and the letter which embodies his conclusions is given below. In accordance with his diagnosis I shall call the intrusive rocks _porphyritic trachyte_; and I am glad to have the weight of his authority in support of my belief that all the rocks of the series are of one type, their resemblances far outweighing their differences.

NOTE.—Persons desiring to examine the Henry Mountain trachytes under the microscope can obtain mounted thin sections from Mr. Alexis A. Julien, School of Mines, Columbia College, New York.

REPORT ON THE LITHOLOGIC CHARACTERS OF THE HENRY MOUNTAIN INTRUSIVES.

BY CAPTAIN C. E. DUTTON.

“I have examined with great interest and attention the Henry Mountain rocks you sent me, and proceed to acquaint you with such results as my limited facilities have permitted me to derive from the examination. It is a very well defined series, having some marked characters which distinguish it from the nearest allied group with which I am acquainted. This is all the more interesting, because I am inclined to think that these peculiarities may have a definable association with or relation to the manner in which the intrusive rocks occur in those “laccolites”, as you term them.

“The hand specimens show in most cases large and unusually perfect crystals of orthoclase imbedded in a very compact uniform paste through which hornblende is also disseminated rather more abundantly than is usually the case where the dominant felspar is monoclinic. Micaceous crystals appear to be wholly wanting and this is a notable circumstance, since the trachytes of the Plateau country, to which these rocks are most nearly allied, are seldom without one or more of them. The only other mineral which is of frequent occurrence in the specimens is magnetite (or possibly titanic iron), which is diffused in the usual form of minute granules in many of them, but is scarce in several of them. In general there is a great scarcity of mineral species and any others than those mentioned are of the greatest rarity.

“The dominant felspar is orthoclase, but a portion of it is triclinic, and I presume this portion is albite, with an occasional occurrence of oligoclase. The groundmass in which the crystals are included is in most cases decidedly compact and without distinguishable crystals, but shows between the crossed Nicols closely aggregated luminous points, which with a ¼-inch objective are resolved indistinctly into felspar. In some cases the crystals of the groundmass are quite apparent with an inch objective, and their species determinable. But the greater portion of the groundmass is quite amorphous, and does not polarize light at all. The proportion of crystalline to amorphous matter in the paste is highly variable—in some cases it is quite bright between crossed Nicols, in others far less so, and in none is it entirely dark. Those specimens which have the finer and more amorphous groundmass have the larger and more perfect crystals of felspar—an association of properties which is not wholly without qualification, but still sufficiently decided.

“Turning to the included felspars, their mode of occurrence is quite an uncommon one, I believe, so far as the eruptive rocks of the Rocky Mountain region are concerned, and give rise to some hesitation before assigning them definitely to the trachytic group. In a great many cases the felspathic crystals are well developed, and so large and so nearly perfect that their aspect is decidedly porphyritic. This is especially the case with the dikes of Mounts Ellsworth and Holmes (Nos. 56, 61, 68, and 69). The orthoclase is invariably of the white “milky” variety, with the exception of a single specimen from a Mount Ellsworth dike (No. 57), where it is present as sanidin. (This is an exceptional rock in all respects, and will be spoken of hereafter.) Nearly all of them appear to have been subject to alteration by chemical action since their formation as is indicated by their diminished power to polarize light. Whether this is due to atmospheric weathering or to changes _en masse_ it is of course impossible to distinguish with certainty, though I incline to the latter view since it is manifested as decidedly in specimens which show no external indications of weathering as in those which do show them. It is not uncommon to find crystals which have almost entirely ceased to polarize. The zonal arrangement is very common in the crystals, and some of the zones contain numberless minute fluid cavities in the largest crystals. The foreign substances included in the crystals present no novelty, being the ordinary films of hornblende, minute needles of felspar, granules of magnetite, and those dust-like points of brownish yellow color which are the proper inclusives of the groundmass.

“The orthoclase occasionally presents the adular variety, but this never becomes a marked feature. I have observed the same in many of the trachytes of the High Plateaus and of the Great Basin. Another phenomenon is the occurrence of crystals which are quite typically monoclinic at one end (orthoclase), and at the other end have the arrangement of plagioclase. This is well known elsewhere, and described by Zirkel. (Mik. Beschaff der Mineralien u. Gesteine).

“In classifying these rocks therefore, we may observe that they present a blending of the characteristics which are common to trachyte and felsitic porphyry. Those who regard porphyry as a distinct class of eruptive rocks would have no hesitation in calling Nos. 18, 56, 61, 68, and 69 undebatable felsitic porphyries, and to the same series might with propriety be added No. 33. With equal confidence Nos. 16, 20, 35, and 43 may be called unqualified trachytes. The other rocks are intermediate in character between these two extremes, and the whole may be regarded as a series in which the individuals form a graduated scale.

“You will recall the fact that many lithologists object to the terms porphyry or porphyritic being used to designate a distinct class or group of rocks, holding that they merely characterize a single feature which is more or less frequently presented by all igneous rocks and having no necessary relation to any of them, and that this is no more adequate to such an important distinction than the color or relative degree of fineness or coarseness of texture. Although this latter view seems to me to underrate the distinctive value of the porphyritic character in general, I incline very decidedly to the belief that it is true as applied to these Henry Mountain rocks. Here at least the porphyritic character has but little significance. In some varieties the crystals are larger and more perfect and the groundmass more homogeneous; in others the crystals are smaller and imperfect, and the groundmass more coarse and irregular; while still others are ‘betwixt and between’. The most careful scrutiny fails to show any fundamental differences in the groundmass or in the included minerals. Hence I think these rocks would be accurately designated _as a group_ by calling them porphyritic trachytes. Such varieties as Nos. 16, 20, and 35 may by themselves be called simply trachyte, the porphyritic character being insufficiently distinct in them to warrant any qualification of the name.

“There is one specimen (No. 57, from dike in Shinarump shale, Mount Ellsworth) which constitutes an exception to the foregoing. By inspection of a hand specimen it might hastily pass for a very compact andesite, but the observer will be instantly undeceived by applying the microscope. It consists of imperfect crystals, which must be sanidin, imbedded in a very close groundmass composed of a material which differs from the foregoing trachytes in being wholly amorphous. Between crossed Nicols the paste transmits no light whatever, though between the parallel Nicols it closely resembles the others. The hand specimen is very dark colored (gray), but the slide is sufficiently translucent. The felspar crystals are either fragmental or very imperfectly developed as to their edges and angles, but polarize very sharply. The amount of twinning is very small, but it appears occasionally. The rock is undoubtedly a trachyte, but an unusual one. Its dark color is not due to hornblende nor to magnetite, both of which occur in it very sparingly, especially the former.

“I have already remarked that the only frequent minerals besides felspar are hornblende and magnetite. Apatite occurs and is tolerably plentiful in a few of the specimens, but absent from most of them. The crystals are all small, requiring a low power for the determination of the larger and a high power for the smaller. They present no peculiarities. Of very rare occurrence is nepheline. This mineral is usually associated with the more basic volcanic rocks and seldom penetrates the trachytic group. Quartz is almost equally rare. The absence of any great variety in the mineral species is quite normal, except possibly the total absence of mica, which is usually present and frequently the only associate of felspar in the western trachytes. The Henry Mountain rocks do not so far as I can discover contain a trace of it.

“In answer to your particular inquiries—

“1st. ‘Is the paste vesicular, or is there any evidence in the crystals to indicate the pressure under which they were formed?’ I can only say that the paste is extremely compact and contains no vesicles even in those specimens of which the aspect is most decidedly trachytic. Some of the larger crystals of felspar contain an abundance of pores or vesicles which may have contained liquids, but with a ¼-inch objective they are too small for treatment by the method you refer to. A few large cavities, usually of irregular shape, occur, but I am quite unfamiliar with the practical treatment of this subject and cannot advise you. I do not find any cavities still containing fluids, and I presume that even if they existed they would be difficult if not impossible to gauge on account of the impellucidity of the felspar. I presume quartz is the most favorable mineral for this investigation on account of its transparency and the greater frequency of its large cavities. Quartz however is almost the scarcest of the contents of these rocks.

“2d. ‘Do any mineralogic differences correlate with the superficial or geographic distribution of the rocks?’ and

“3d. ‘Do any mineralogic differences correlate with the vertical distribution of the trachytes?’ As it appears from your account of the distribution of the masses that the upper zone belongs (with one exception) to Mounts Ellen and Pennell and the lower to the other mountains, the answer to one is the answer to both, and this is in the negative. The mineralogical differences are exceedingly small, considering the number of distinct masses, and this covers the inquiry entirely. Regarding the texture or habitus, on the contrary, it appears to me that the true trachytes predominate in the upper zone and the porphyries in the lower, but not without exceptions.

“4th. ‘Do any mineralogic differences correlate with the size of the intrusive masses?’ No mineralogical differences thus correlate, but I find a preponderance of the porphyritic texture in the smaller masses and of the trachytic texture in the larger. It is not without exception, and the preponderance is small.”

_Metamorphism and Contact Phenomena._—Wherever the trachytes came in contact with the sedimentaries the latter were more or less altered. Large bodies of trachyte produced greater changes than small. The laccolites both metamorphosed their walls more completely, and carried their influence to a greater distance than the sheets and dikes. The summits of the laccolites had a greater influence than the edges; a phenomenon to which I shall have occasion to revert. The sandstones were less affected than the shales, at least in such characters as readily catch the eye. Clay shales were indurated so as to clink under the hammer, and Captain Dutton discovered with the microscope that minute crystals of felspar had been developed. Sandstones were usually modified in color, and their iron was segregated so as to give a mottled or speckled appearance to the fracture. They were indurated, but the granular texture was always retained.

The trachyte carries numerous small fragments of sedimentary rock broken apparently from its walls, and these are as thoroughly crystalline as their matrix.

The altered rocks are usually jointed, but nothing approaching to slaty cleavage was seen, nor has there been any crumpling.

The reciprocal influence of the sandstone and shale upon the trachyte was small. Specimens broken from the contact surface of a laccolite and from its interior cannot be distinguished. In the Marvine laccolite however there is a difference between the exterior and interior portions in their ability to withstand erosion.

_Historical._—Before leaving the subject of the structure of the mountains it is proper to place on record certain observations by others which antedated my own but have never been published.

While Professor Powell’s boat party was exploring the cañons of the Colorado, Mr. John F. Steward a geologist and member of the party climbed the cliff near the mouth of the Dirty Devil River and approached the eastern base of the mountains. He reported that the strata had in the mountains a quaquaversal dip, rising upon the flanks from all sides.

The following year Prof A. H. Thompson then as now in charge of the geographic work of Professor Powell’s survey crossed the mountains by the Penellen Pass and ascended some of the principal peaks. He noted the uprising of the strata about the bases and the presence of igneous rocks.

In 1873 Mr. E. E. Howell at that time the geologist of a division of the Wheeler Survey traveled within twelve miles of the western base of the mountains, and observed the uprising of the strata.

My own observations were begun in 1875, at which time a week was spent among the mountains. They proved so attractive a field for investigation that in the following year a period of nearly two mouths was devoted to their study.

_Other Igneous Mountains._—The Henry Mountains are not the only igneous group which the Plateau province comprises. They are scattered here and there throughout its whole extent. From the summits of the Henry Mountains one can see the Sierra La Sal ninety miles to the northeastward, and the Sierra Abajo seventy miles to the eastward. Beyond them and two hundred miles away are the Elk Mountains of Colorado. Fifty miles to the southwestward stands the Navajo Mountain on the brink of the Colorado; and one hundred and twenty miles to the southeast the Sierra La Lata and the Sierra Carriso are outlined against the horizon. Westward it is less than thirty miles to the Aquarius Plateau, the nearest member of the great system of volcanic tables among which the Sevier and Dirty Devil Rivers rise.

Beyond the horizon at the south and southwest and southeast are a series of extinct volcanoes; Mount Taylor and the Marcou Buttes in New Mexico; the Sierra Blanca, the Sierra Mogollon, the San Francisco Group and the Uinkarets in Arizona; and the Panguitch Lake Buttes in Utah.

Of the groups which are visible, all but that of the Aquarius Plateau are allied in character to the Henry Mountains.

The Sierra Abajo was studied in 1859 by Dr. J. S. Newberry, geologist of the Macomb Expedition, who writes: “Within the last few weeks we have been on three sides of this sierra, and have learned its structure quite definitely. It is a mountain group of no great elevation, its highest point rising some 2,000 feet above the Sage-plain, or perhaps 9,000 feet above the sea. It is composed of several distinct ranges, of which the most westerly one is quite detached from the others. All these ranges, of which there are apparently four, have a trend of about 25° east of north, but being arranged somewhat _en echelon_, the most westerly range reaching farthest north, the principal axis of the group has a northwest and southeast direction. The sierra is composed geologically of an erupted nucleus, mainly a gray or bluish-white trachyte, sometimes becoming a porphyry, surrounded by the upheaved, partially eroded, sedimentary rocks. The Lower Cretaceous sandstones and Middle Cretaceous shales are cut and exposed in all the ravines leading down from it, while nearly the entire thickness of the Cretaceous series is shown in spurs which, in some localities, project from its sides; apparently the remnants of a plateau corresponding to, and once connected with, the Mesa Verde. Whether the Paleozoic rocks are anywhere exposed upon the flanks of the Sierra Abajo I cannot certainly say, though we discovered no traces of them. It is, however, probable that they will be found in some of the deeper ravines, where, as in most of these isolated mountains composed mainly of erupted material, they are doubtless but little disturbed, but are buried beneath the ejected matter which has been thrown up through them.

“The relations of the Cretaceous rocks to the igneous nucleus of the Sierra Abajo are very peculiar, for, although we did not make the entire circuit of the mountain mass, and I can, therefore, not speak definitely in regard to the western side, as far as our observations extended we found the sedimentary strata rising on to the trachyte core, as though it had been pushed up through them.” (Geology of the Macomb Expedition, page 100.)

Of another group Dr. Newberry says in the same report (page 93): “Of the composition of the Sierra La Sal we know nothing except what was taught by the drifted materials brought down in the cañons through which the drainage from it flows. Of this transported material we saw but little, but that consisted mainly of trachytes and porphyry, indicating that it is composed of erupted rocks similar to those which form the Sierra Abajo, of which it is in fact almost an exact counterpart. From the cliffs over Ojo Verde we could see the strata composing both the upper and second plateaus, rising from the east, south, and southwest on to the base of the Sierra La Sal, each conspicuous stratum being distinctly traceable in the walls of the cañons and valleys which head in the sierra. It is evident, therefore, that the rocks composing the Colorado Plateau are there locally upheaved, precisely as around the Sierra Abajo * * *.”

These mountain groups have been since visited by the geologists of Dr. Hayden’s survey, Dr. A. C. Peale ascending the Sierra La Sal and Mr. W. H. Holmes the Sierra Abajo. Mr. Holmes has also examined the La Lata and Carriso Mountains and found in them the same upbending of Cretaceous strata and the same association of igneous material.

The Navajo Mountain has been viewed by Mr. Howell and by the writer from a commanding position on the opposite side of the Colorado River, and fragments of its trachyte have been gathered on the river bank by Professor Powell, but no geologist has yet climbed it. Still there can be no question of its general structure. It is a simple dome of Jura-Triassic sandstone, springing abruptly from a plateau of the same material, and veined at the surface by sheets and dikes of trachyte—the counterpart in fine of Mount Ellsworth, only of more imposing proportions.

The La Sal, the Abajo, the La Lata, the Carriso, the Navajo, and the Henry Mountains agree in their essential features. Structurally they have no trends. Their phenomena are grouped about centers and not axes. In all of them the strata are lifted into dome-like arches, and associated with these arches are bodies of trachyte. The trachytes are all of one lithologic type, and are so closely related that a collection of rock specimens representing all the groups would show scarcely more variety than a collection representing the Henry Mountain laccolites. With so many characters in common they can hardly fail to agree in the possession of laccolitic nuclei.[3]

Footnote 3:

While these pages are passing through the press a paper by Dr. A. C. Peale “On a peculiar type of eruptive mountains in Colorado” (Bulletin U. S. Geol. Sur., Vol. III, No. 3) comes to hand. He groups together as of one type not only the Elk, La Sal, Abajo, La Lata, and Carriso Mountains, but also the Spanish Peaks, Park View Mountain, Mount Guyot, Silverheels Mountain, the San Miguel Mountains, the La Plata Mountains, and certain smaller masses in Middle Park and near the Huerfano River. He says, “Although modified in several instances, the general plan appears to be the same. The igneous material came up through fissures in the sedimentaries, sometimes tipping up their ends, and sometimes passing through without disturbing them. On reaching the Cretaceous shales, it generally spread out in them, and pushed into and across them dikes and intrusive sheets of the same igneous rock. The elevation in some cases appears to be due to actual upheaval caused by the eruptive force. The mountains as they now exist are doubtless largely the result of erosion, the hard igneous rock opposing greater resistance to erosive influences than do the surrounding soft sedimentary beds.”

The Elk Mountains are at the very margin of the plateau, and geographically might be connected with the Sawatch Range which bounds the plateau province on that side. But structurally they are a group instead of a range, and affiliate with the groups which are insulated by an environment of tables. Thanks to the labors of Mr. Holmes and Dr. Peale their general structure is known. The Eastern Elk Mountains consist of four great bodies of “eruptive granite”, over which are arched not only Mesozoic but Paleozoic strata. Their foundation must be a floor of Archæan metamorphics. Two of them, the Snow Mass and White Rock laccolites, are joined by a continuous line of disturbance, in the description of which by pen and pencil Mr. Holmes has made an important contribution, not only to dynamical geology, but to the methods of geological illustration. The others are more symmetric and are complementary illustrations of the common structure. One, the Sopris, is half truncated by erosion so that the core is exposed at top with an encircling fringe of upturned sedimentaries; and the other, the Treasury, retains a complete arch of Paleozoic strata. The Western Elk Mountains are a cluster of smaller laccolites which are inserted between strata of Cretaceous age. Their traps include porphyritic trachytes undistinguishable from those of the Henry Mountains, and eruptive granites identical with those of the Eastern Elk Mountains; and they exhibit a gradation from one to the other. Indeed the two rocks are nearly related, and their assignment to classes so diverse as trachyte and granite is merely an illustration of the imperfection of our classification of rocks. The description of the Elk Mountains will be found on pages 61 to 71 and 163 to 168 of the Annual Report for 1874 of the “Geological and Geographical Survey of the Territories.”

If we turn now to the distinctively volcanic mountains of the Plateau province—to those which are built by eruption at the surface—we leave at once the porphyritic trachytes. Mount San Francisco, Mount Bill Williams, Mount Sitgreaves, Mount Kendrick, Mount Floyd and the Sierra Blanca (of Arizona) are all composed of basic trachytes, and so are the Aquarius Plateau and the many tables that lie beyond it.

The Mogollon group, the Marcou Buttes, the minor cones about Mount San Francisco, the Uinkarets, and the Panguitch Lake group are basaltic. In each of these instances the igneous rock issued above the surface and there is no evidence by displacement that any portion of it was deposited below.

Mount Taylor may be an exception. In the character of its lava and its general features it resembles Mount San Francisco, but there are disturbed strata on its southern flank, and it is possible the mountain is both extrusive and intrusive. Extrusion and intrusion are probably combined in some small tables lying fifty miles north of the Henry Mountains. They are built of Flaming Gorge shale, preserved from erosion by dikes, sheets, and (probably) outflows of basalt.

Combining all these facts we attain to a simple relation between two types of igneous rock on the one hand, and two types of mountain structure on the other. One type of rock is acidic, including “porphyritic trachyte” and “eruptive granite”, and its occurrence is without exception intrusive. The other type of rock is basic, including basic trachyte and basalt, and its occurrence is almost uniformly extrusive.

It is not possible to combine the two groups of phenomena by saying that in one case the eruptive cones cover laccolites, and in the other the laccolites have been covered by eruptive cones which have disappeared; first, because many of the eruptive cones are too well exposed to admit of the concealment of laccolitic arches beneath them; second, because the two types of lava are essentially different. The acidic type if extruded at the surface would be an ordinary trachyte; the basic type if crystallized under pressure would be classed with the greenstones.

The basis for the generalization is exceedingly broad. I have enumerated only seven groups of laccolitic mountains and ten groups of eruptive; but with few exceptions each group is composed of many individuals, each one of which is entitled to rank as a separate phenomenon. In the Uinkaret Mountains Professor Powell has distinguished no less than one hundred and eighteen eruptive cones, and in the Henry Mountains I have enumerated thirty-six individual laccolites. In one locality basic lava has one hundred and eighteen times risen to the surface by channels more or less distinct, instead of opening chambers for itself below. In the other locality porphyritic trachyte has thirty-six times built laccolites instead of rising to the surface.

If our attention was restricted to these two localities we might as naturally correlate the types of structure with some accidents of locality as with types of lava; but when all the localities are taken into account it is evident that there is no common mark by which either the laccolitic or the volcanic are distinguished.

THE QUESTION OF CAUSE.

We are now ready to consider the question: Why is it that in some cases igneous rocks form volcanoes and in other cases laccolites?

It is not necessary to broach the more difficult problem of the source of volcanic energy. We may assume that molten rock is being forced upward through the upper portion of the earth’s crust, and disregarding its source and its propelling force may restrict our inquiry to the circumstances which determine its stopping place.

Let us further assume, but for a moment only, that the cohesion of the solid rocks of the crust does not impede the upward progress of the fluid rock, nor prevent it from spreading laterally at any level. The lava will then obey strictly the general law of hydrostatics, and assume the station which will give the lowest possible position to the center of gravity of the strata and lava combined.

(1) If the fluid rock is less dense than the solid, it will pass through it to the surface and build a subaërial mountain.

(2) If the upper portion of the solid rock is less dense than the fluid, while the lower portion is more dense, the fluid will not rise to the surface but will pass between the heavy and light solids and lift or float the latter.

(3) If the crust be composed of many horizontal beds of diverse and alternating density, the fluid will select for its resting place a level so conditioned that no superior group of successive beds, including the bed immediately above it, shall have a greater mean specific gravity than its (the fluid’s) own; and that no inferior group of successive beds, including the bed immediately beneath, shall have a less mean specific gravity than its own.

[Illustration:

FIG. 51.—Diagram to illustrate the application of the law of Hydrostatic Equilibrium to the movements of lavas. The shaded bands represent heavy strata; the open, light. ]

[In the diagram, a series of light and heavy beds are represented in section by open and shaded spaces. A lava stream free to move upward or laterally will intrude itself at some point (_c_) so placed that every combination of superior beds (_a_), which includes the lowest, shall have a less average density; and every combination of inferior strata (_b_), which includes the highest, shall have a greater average density than that of the lava.]

The first case is that of a volcano; the second is that of a laccolite; and the third is the general case, including the others and applying to all volcanoes and laccolites.

Conversely we may say that, given a series of strata of diverse and alternating density, a very light lava will traverse it to the top and be extruded; a heavier will intrude itself at some lower level; and a series of dissimilar lavas may select an equal number of distinct levels.

It is easy to imagine such a balancing of conditions that a slight change in a lava will determine a great change in its level of intrusion.

Having seen the general application of the hydrostatic law, it is time to recall the condition which we laid aside at the start. Cohesion, or rigidity, is never absent and must affect every phase of vulcanism. It certainly opposes the free circulation of lavas, and it cannot but modify their obedience to the hydrostatic law.

But granting this, and believing that a full comprehension of the subject must include this condition, I am at a loss to tell in what way it influences the selection by a lava flood of a subaërial or a subterranean bourne. Whether it will on the whole oppose upward progress more than lateral, or _vice versa_, is not clear. If it resists lateral intrusion the more strongly, it favors the formation of volcanoes; if it resists upward penetration the more strongly, it favors the formation of laccolites; and in either case the working of the hydrostatic law is modified.

But in neither case is the working of the law more than modified. The law is not abrogated, and in obedience to it light lavas still _tend_ to rise higher than heavy however much the rising of all lavas may be hindered or favored.

In brief, since lavas are fluids they are subject to the law of fluid equilibrium, and their behavior is conditioned by the relations of their densities to the densities of the solids which they penetrate; and since the latter solids are rigid and coherent, it is further conditioned by the resistance which is opposed to their penetration. When the resistance to penetration is the same in all directions, the relation of densities determines the stopping place of the rising lava; but when the vertical and lateral resistances are unequal, _their_ relation may be the determining condition.

If we can decide whether the determinative condition in the Plateau region was that of densities or that of penetrability, we shall have solved our problem.

Assuming, first, that the essential condition is that of penetrability, we should expect that some particular stratum or that a few particular strata, being less penetrable than others, would check the rising lavas and accumulate them in a system of laccolites, which would occupy one, or a few definite horizons. Volcanoes would occur in districts from which such impenetrable strata either were originally absent or had been removed before the igneous epoch; and we should expect to find the same variety of material in laccolites and in volcanoes.

Assuming, second, that the essential condition is that of densities, we should expect as before to find certain stratigraphic horizons more favorable than others to the accumulation of laccolites, and we should also expect to find certain lavas usually volcanic and certain others usually laccolitic.

That is to say—since the condition of impenetrability resides in the solid rock only, and the condition of density pertains to both solid and fluid, either condition might determine laccolites at certain stratigraphic horizons, while the latter only could discriminate certain lavas as intrusive and others as extrusive.

The vertical distribution of laccolites is not inconsistent with either assumption. In the Henry Mountains there are two zones of occurrence; in the Eastern Elk Mountains there is a third; and it is probable in the present state of our knowledge that all other laccolites of the Plateaus can be assigned to one or another of these. The fact that the laccolites of the upper zone have a vertical range of two thousand feet is rather favorable to the idea that their stations were determined by relations of density, but is not decisive.

When however we turn to the relation between the constitutions and the behaviors of lavas, we find the entire weight of the evidence in favor of the assumption that conditions of density determine the structure. The coincidence of the laccolitic structure with a certain type of igneous rock is so persistent that we cannot doubt that the rock contained in itself a condition which determined its behavior.

We are then led to conclude that the conditions which determined the results of igneous activity were the relative densities of the intruding lavas and of the invaded strata; and that the fulfillment of the general law of hydrostatics was not materially modified by the rigidity and cohesion of the strata.

Having reached this conclusion it is natural to seek for confirmation by the investigation of the densities of the rocks concerned in the phenomena. As will appear by a table given further on, the density of the Henry Mountain trachyte has been determined to be 2.61.; but the densities of the erupted lavas of the Plateaus are not yet known. There can be no doubt however that the latter are heavier. Von Cotta in his Lithology gives 2.9 to 3.1 as the density of basalt, and 2.6 to 2.9 as the density of the more basic trachytes. And in general, it is well established that where the state of aggregation is the same, basic igneous rocks are always heavier than acidic. But in order that the laccolitic structure should have been determined by density, the acidic rock of the laccolites must have been heavier in its _molten_ condition than the more basic rocks of the neighboring volcanoes; and since in the _crystalline_ condition the acidic is the lighter, it follows that it has gained less density in cooling than the basic.

If the amount of contraction of the several rocks in passing from their natural molten condition to the crystalline condition could be determined experimentally, a crucial test would be applied to our conclusions as to the origin of laccolites. The matter is however beset with difficulties. Bischof attempted by melting eruptive rocks in clay crucibles to obtain their ratios of expansion and contraction, but his method involved so many sources of error that his results have been generally distrusted. He concluded that the contraction in passing from the molten to the crystalline state is greater in acidic than in basic rocks. Delesse by an extended series of experiments in which crystalline rocks were melted and afterward cooled to glasses, showed that acidic rocks increase in volume from 9 to 11 per cent. in passing from the crystalline state to the vitreous, while basic increase only 6 to 9 per cent. Mallet concluded from some experiments of his own that the contraction of rocks in cooling from the molten condition is never more than 6 per cent., and that it is greater with basic than with acidic rocks; but considering that the substances which he treated were artificial and not natural products, that his methods were not uniform, and that he ignored the distinction between the vitreous and the crystalline, of which Delesse had demonstrated the importance, no weight can be given to his results.

If however all of these experiments were trustworthy and their results were concordant, their bearing upon the problem of the laccolites would still be slight. It is generally conceded that the fusion of lavas is hydrothermal, while in all the experiments recourse was had to dry fusion; and the densities attained in the two ways are necessarily different. The practical difficulty in the way of restoring the natural molten condition is great and may be insuperable, but unless it shall be overcome we cannot learn experimentally the changes of density which igneous rocks undergo in congelation.

There is a fact of observation which tends to sustain the view that the laccolitic rocks contracted less in cooling than the volcanic. The prismatic structure is produced by the contraction of cooling rocks during and after solidification. That it does not occur in the Henry Mountain trachytes indicates that their contraction was small. That it does occur at numerous localities in Utah in basalts, indicates that their contraction was relatively great. Mr. Jukes, in his Manual of Geology, says that it is most frequently exhibited in “doleritic lavas and traps, being especially characteristic of basalt, but occurs almost as perfectly in some greenstones and felstones”; and in the range of my own observation I can recall no instance of its occurrence in other than basic rocks.

For the sake of comparing the densities of the intrusive rocks with those of the strata which contain them, a number of determinations were made of the specific gravities of specimens representative of the trachytes and of the several sedimentary groups of the Henry Mountains.

Trachytes were selected to represent as great a variety of locality and relation as possible, and at the same time exclude all specimens which showed traces of decomposition. Hand specimens weighing from one hundred to four hundred grains were used, and these were weighed first dry, and then suspended in water. By using such large quantities averages were obtained of a rock which, minutely considered, is heterogeneous; and by using the blocks entire instead of pulverized or granulated, the state of aggregation of its minerals was included as an element of the specific gravity of the rock.

It will be observed that the range, 2.54 to 2.66, is very small.

_Table of Specific Gravities of Trachytes of the Henry Mountains._ ───────────────────────────────────────────────────┬───────────────── Locality. │Specific gravity. ───────────────────────────────────────────────────┼───────────────── East flank of Mount Pennell; sheet │ 2.66 Marvine Laccolite; north base of Mount Ellen │ 2.65 Peale Laccolite; east flank of Mount Ellen │ 2.64 Dike on Mount Ellsworth │ 2.64 South base of Mount Hillers; sheet │ 2.63 Sheet under the Peale Laccolite │ 2.62 Scrope Laccolite; southeast base of Mount Ellen │ 2.60 Bowl Creek Laccolite; northeast base of Mount Ellen│ 2.58 North spur of Mount Pennell; dike │ 2.58 Sentinel Laccolite; north base of Mount Pennell │ 2.54 │ ———— Mean │ 2.61 ───────────────────────────────────────────────────┴─────────────────

Specimens to represent the stratigraphic series were selected at the _margins_ of the disturbed region so far as possible, to avoid the effect of metamorphism. But as it was not practicable to eliminate this source of error in every case, the densities of highly metamorphic specimens were also measured for the purpose of indicating the effect of the metamorphism. In order to restore so far as practicable the condition of the rocks at the time of the lavic intrusion, the specimens were saturated with water, and in this condition were weighed in air as well as in water. The results for the porous sandstones are from one-seventh to one-fourteenth lower than would have been obtained by the usual method. Hand specimens were used as before.

_Table of Specific Gravities of Sedimentary Rocks of the Henry Mountains._ ──┬─────────────────────────────────┬────────────────┬───────────────── │ Rock. │ Condition. │Specific gravity. ──┼─────────────────────────────────┼────────────────┼───────────────── 1│Masuk Sandstone │Unaltered │ 2.16 2│Blue Gate Sandstone │Unaltered │ 2.14 3│Blue Gate Shale │Unaltered │ 2.45 4│Flaming Gorge Shale │Unaltered │ 2.42 5│Gray Cliff Sandstone │Unaltered │ 2.13 6│Vermilion Cliff Sandstone, (top) │Unaltered (?) │ 2.21 7│Vermilion Cliff Sandstone, (base)│Unaltered (?) │ 2.28 8│Henry’s Fork Conglomerate │Slightly altered│ 2.25 9│Vermilion Cliff Sandstone, │Altered │ 2.48 10│Aubrey Sandstone │Altered │ 2.55 11│Tununk Shale │Altered │ 2.60 ──┴─────────────────────────────────┴────────────────┴─────────────────

It is plain from this table that the effect of the metamorphism was to increase the densities of the rocks affected. The Blue Gate shale which _unaltered_ gave 2.45, is lithologically identical with the Tununk shale which _altered_ gave 2.69. The Aubrey sandstone cannot be observed unaltered in the vicinity of the mountains, but at a distance of forty miles where it again comes to the surface it closely resembles the Gray Cliff sandstone. If it has the same normal weight as the latter, then it has increased from 2.13 to 2.55.

The specimens of the Vermilion Cliff sandstone numbered 6 and 7 were not visibly changed, but as they were obtained from the flank of the Holmes arch there was reason to suspect that their condition was not normal, and the determined densities strengthen the suspicion. Judged by other localities, the normal density of the Vermilion Cliff rock is not far from that of the Gray Cliff rock, namely 2.13; and it is easy to believe that the upper portion of the bed where it lay on the side of the Holmes arch was changed in density to 2.21; while the lower portion lying nearer the laccolite was changed to 2.28; and while the same bed among the Ellsworth dikes acquired the density of 2.48.

Taking into account both these considerations and certain others which need not be enumerated, I derive the following:

_Table of the Specific Gravities of the Henry Mountain Sedimentary series in the Order of Superposition._ ───────────────────────────────────┬───────────────── Bed. │Specific gravity. ───────────────────────────────────┼───────────────── Masuk Sandstone │ 2.16 Masuk Shale estimated│ 2.40 Blue Gate Sandstone │ 2.14 Blue Gate Shale │ 2.45 Tununk Sandstone │ 2.15 Tununk Shale estimated│ 2.45 Henry’s Fork Conglomerate │ 2.25 Flaming Gorge Shale │ 2.42 Gray Cliff Sandstone │ 2.13 Vermilion Cliff Sandstone estimated│ 2.15 Shinarump Shale estimated│ 2.40 Aubrey Sandstone estimated│ 2.15 ───────────────────────────────────┴─────────────────

Taking into account the thicknesses of the several beds enumerated in the foregoing table, it is easy to obtain the mean specific gravity of all which lie above a given horizon; and by making this determination for the horizon of the base of each of the indicated beds, the following table has been derived. The figures are based on the assumption that the rock series included nothing above the Masuk sandstone. If (as is probable) there were Tertiary beds also, the estimates are too low, for the Tertiaries of the vicinity are calcareous and argillaceous and consequently dense.

_Table showing the Mean Specific Gravities of the Rock Series contained between certain horizons and the summit of the Masuk Sandstone._ ─────────────────────────────────┬───────────────── Horizons. │ Specific │ gravities. ─────────────────────────────────┼───────────────── Base of Masuk Sandstone │ 2.16 Base of Masuk Shale │ 2.28 Base of Blue Gate Sandstone │ 2.23 Base of Blue Gate Shale │ 2.32 Base of Tununk Sandstone │ 2.31 Base of Tununk Shale │ 2.34 Base of Henry’s Fork Conglomerate│ 2.33 Base of Flaming Gorge Shale │ 2.36 Base of Gray Cliff Sandstone │ 2.33 Base of Vermilion Cliff Sandstone│ 2.32 Base of Shinarump Shale │ 2.33 ─────────────────────────────────┴─────────────────

From this it appears that the laccolites of the upper zone, extending from the lower part of the Blue Gate Shale to the upper part of the Flaming Gorge Shale, bore loads of which the mean densities were from 2.31 to 2.34, and that laccolites of the lower zone, which has its upper limit in the Shinarump Shale, bore loads of which the mean densities were 2.32 and upward. If the positions of the laccolites were determined purely by the law of hydrostatic equilibrium, then these figures define the density of the molten trachyte, and show that its contraction in cooling—from the density 2.34 to the density 2.61—was about one-tenth of its volume.

THE STRETCHING OF STRATA.

It has been the opinion, not only of the writer but of other students of the displacements of the West, that the ordinary sedimentary rocks, sandstone, limestone, and shale, are frequently _elongated_ as well as compressed by orographic movements, and that this takes place without any appreciable metamorphism; but it is difficult to find opportunity for the demonstration of the phenomenon by measurement. When a fold is made in a level stratum, either of two things may take place; the portions of the stratum which remain level at the sides may approach each other; or the stratum may be stretched. But when a circular portion of a continuous level stratum is lifted into a quaquaversal arch (as illustrated in Figure 11), an approach of the level portions is out of the question, and there must be a stretching or a fracture. Of the unfractured quaquaversals of the Henry Mountains there is one which combines all the essentials of a crucial case. The Lesser Holmes arch is nearly isolated; on three sides it rises from the undisturbed plateau, and on the fourth it joins a similar but fractured dome. The major part of its surface is composed of one bed, the Vermilion Cliff sandstone, broken only by erosion. Comparing the length of this bed in its present curved form with the space it must have occupied before it was upbent, I find that in a distance of three miles it has been elongated three hundred feet. Moreover there is every reason to suppose that the elongation was produced quickly, or at least by a succession of finite rather than infinitesimal increments; for the lifting of the arch was caused by the intrusion of a laccolite, and though the latter may have been built by the addition of many separate lava flows, it could not have risen with secular and continuous slowness. The molten trachyte, rising through a passage and into a reservoir that were comparatively cool, would have clogged itself by congelation had it not moved with a certain degree of rapidity.

[Illustration:

FIG. 52.—Cross-section of an uplifted dome. The dotted lines show the original position of a bed; the curved lines, the imposed. ]

The condition which rendered possible the elongation and the sudden bending of so rigid and brittle a rock as a massive sandstone, was pressure. At the time of the uplift the sandstone was buried by other sediments to a depth of from five thousand to eight thousand feet, and sustained a pressure of from five thousand to eight thousand pounds to the square inch. Now the experiments which have been made upon building stones show that the weight required to crush similar sandstones in a dry condition, is three thousand to five thousand pounds to the inch; and it is a fact familiar to quarrymen that sandstone and limestone which are quarried below the water level are both softer and weaker while they are still saturated than they are after drying. So we may fairly assume that the Vermilion sandstone was loaded at the time of its displacement with a crushing weight. No part could yield to the pressure while it was sustained by the surrounding parts; but every part was ready to yield whenever its support was withdrawn. It was in a quasi-plastic state and abhorred a fissure as strongly as “nature abhors a vacuum”, and for the same reason. A fissure could not be opened in it unless it was coincidently filled by something—such as lava—which would resist the tendency of its walls to flow together. The formation of a gaping fissure being thus prevented, and the uplifting of the dome requiring that the sandstone should cover a greater area, an extension of the bed was the necessary result. It was not _stretched_ into the dome form; it was _compressed_. The efficient force did not act in the direction of the extension, but vertically. The sandstone was pushed, not pulled.

If this explanation is the true one, then it is true in general that just as for each rock there is a crushing weight, so there is for each rock a certain depth at which it cannot be fissured and can be flexed. The softer rocks are plastic at small depths. Fire-clays under coal seams exude, or “creep”, even with the pressure of a few feet of superincumbent strata. Springs of water rise at the outcroppings of soft strata because the joints which intersect most rocks near the surface of the ground cannot cross those which are soft enough to yield under the pressure incident to them. If the soft beds were jointed they would not intercept percolating water, and the distribution of springs would be very different.

The phenomena of fissure veins are in point. When a fault takes place, and one rock mass is slidden past another to which it had been joined it is usually the case that the opposed surfaces no longer fit together as they did before the movement, and interspaces are left. These become filled, at first by water, and afterward by minerals deposited from the water, and the mineral masses thus deposited are called fissure veins. But the preservation of the interspaces depends upon the rigidity of the rocks which inclose them; and it frequently happens that where a system of rocks is traversed by a fault, the harder will keep somewhat apart and maintain a fissure, while the softer will be crushed together without an interspace. If the mineral vein which forms in such a fissure is afterward explored in mining, it is found to be traceable and continuous so far as it is walled by the hard rock upon both sides, but when the hard is replaced by the soft in one or both walls, the vein is either reduced to a mere fillet or disappears completely. If the fault extends to a great depth, it will finally reach a region where the hardest rocks which it separates are coerced by so great a pressure that they cannot hold themselves asunder, but are forced together before the fissure can be filled by mineral deposits. Thus there is a definable inferior limit to the region of vein formation; and even while it is impossible to assign a downward limit to the fault which made place for a vein, it may be possible to assign a downward limit to the vein itself.

Accordant with this view is the absence of fissure veins from the Henry Mountains. Displacement and thermal disturbance are usually regarded as the conditions of mineral concentration; and here were displacement and lavic intrusion coincident in time and place. The heat which metamorphosed great bodies of shale and sandstone was surely competent to excite the currents and reactions which concentrate minerals in veins; but the displacements did not open fissures, and the heated water could circulate only through the pores of rocks. Fissure veins were impossible, and the sluggish currents which were engendered in continuous rock masses did not effect a great change in the distribution of minerals.

THE CONDITIONS OF ROCK FLEXURE.

There are three known conditions under which strata of the most rigid character may be bent without fracture; or in other words there are three ways in which flexibility may be either induced or demonstrated. At ordinary temperatures and at the surface of the earth a hard stratum cannot be quickly flexed. But no rigidity is absolute, and a constant strain, even though slight, will in the course of time produce deformation. The same result may be accomplished quickly if the temperature of the stratum is raised to near the point of fusion. Or it may be accomplished with neither great heat nor great time if only the stratum is so deeply buried that the weight of its cover keeps it from opening fissures. The three conditions of flexure are time, heat, and pressure; and whenever the circumstances of a displacement include none of these, the rocks are broken. A fourth condition, moisture, is of great importance as an accessory, but alone it is not sufficient to prevent fracture. The whole body of strata of the earth’s crust is saturated with water, except a very little at the surface, and all rock movements are thereby facilitated. If the strata were dry, their flexure would require much more time, or heat, or pressure, than is necessary in their moist condition.

Often the three conditions complement each other; but not always.

We may say, the greater the load which strata bear the more rapidly they can be flexed; and conversely, the more slowly strata are displaced the less the pressure necessary to prevent fracture.

And we may say, the higher the temperature of strata the more rapidly they can be flexed; and conversely, the more slowly strata are displaced the lower the temperature necessary to prevent fracture.

For both these statements we find support in a great series of homologies. But we cannot affirm that such a reciprocal relation exists between the effects of heat and pressure. For all rocks are believed to expand by heating, up to the point of fusion; and it is a recognized physical law that in all bodies which heat expands, the effects of heat are opposed by pressure. Hence we cannot say, “The heavier the load which strata bear the lower the temperature necessary to prevent fracture”, nor can we say, “The higher the temperature of strata the less the load necessary to prevent fracture”.

THE QUESTION OF COVER AND THE QUESTION OF AGE.

It is evident that the laccolites of the Henry Mountains were formed beneath the surface of the earth’s crust, but at what depth is not so evident. The problem is involved with the problem of the age of the laccolites, and the two are connected with the general history of the Basin of the Colorado. Neither problem can be called, for the present at least, determinate, but it is possible to narrow them down by the indication of limits which their solutions will not exceed.

So much of the Colorado Plateau region as lies within Colorado and Utah was covered during a geological age which it is convenient to call Cretaceous, by a sea, the waters of which appear to have become fresh toward the last. Then came elevation both general and differential. A great part of the sea bed became dry land, and the accumulated sediments together with many which underlay them were bent into great waves thousands of feet in altitude. The crests of the waves were subjected to erosion and truncated. Then came a second submergence which was purely lacustrine. In some way that has not been ascertained a lake basin was formed, and the region received a new system of sediments which it is convenient to call Tertiary, and which not merely filled the troughs between the great rock-waves but covered the truncated summits of the waves themselves. Then followed the desiccation of the basin by the cutting down of its rim where the water overflowed. The overflowing river as it deepened its channel and gradually lowered the lake, steadily extended its upper course to follow the receding shore; and finally when the basin was completely drained the river remained, its channel leading through what had been the deepest part of the Tertiary sea. That river is the Colorado. As portions of the lake bottom were successively drained they began at once to be eroded, and from that time to this there has been progressive degradation. The regions nearest to the central river were reduced most rapidly and have been completely stripped of their Tertiary strata, but broad areas of the latter remain at the west, and north, and east.

(The reader will understand that this succinct history is shorn for the sake of clearness of all details and qualifications. There have been complicating eruptions and displacements or oscillations at every stage, and if the full story could be told, it would not be by a single paragraph nor by a single chapter.)

When the Cretaceous strata were thrown into waves the site of the Henry Mountains remained in a trough, and it probably was not dried, but continued the scene of sedimentation while the crests of the surrounding rock-waves were worn away. Certainly it was not greatly eroded at that time; and when the Tertiary lake beds were thrown down it was favorably disposed for a heavy deposit. It is not extravagant to assume that four thousand feet of lake beds rested on the Masuk sandstone at the beginning of the final desiccation.

In brief there may be distinguished—

1. The deposition of the Cretaceous.

2. The folding and erosion of the Cretaceous.

3. The deposition of the Tertiary.

4. The desiccation of the Tertiary lake basin.

5. The erosion which is still in progress.

It is evident that the laccolites were not formed until the Cretaceous strata had been deposited; for their uplifts have bent and tilted all Cretaceous rocks up to and including the Masuk sandstone.

They were not formed at any late stage of the final erosion, for they conserve tables along their western base, which but for their shelter would long since have disappeared. From the end of the Cretaceous period to the end of the desiccation of the basin there is no event with which the laccolites can be directly connected. There is however a consideration which in an indirect way sanctions the opinion that the epoch of igneous activity was after the deposition of the Tertiaries and before their erosion.

The Masuk Sandstone is at once the summit of the Cretaceous and the highest bed in the present Henry Mountain section. If it were restored over the entire range, the laccolites of the upper zone would have on the average thirty-five hundred feet of cover, and those of the lower zone nearly seven thousand feet. This was the depth of their original cover, if they were intruded at the close of the Cretaceous age. During the epoch of Tertiary deposition and the subsequent epoch of erosion, the cover first increased in depth and then diminished, having its maximum at the end of the Tertiary deposition. If it can be shown that the original cover of the upper laccolites exceeded thirty-five hundred feet, the question of age will be reduced to comparatively narrow limits. In order to discuss the problem of the original depth of cover it will be necessary to consider another matter, of which the connection will not at first be apparent.

_The size of laccolites._—It is a matter worthy of note that no laccolite of inconsiderable extent is known in the Henry Mountains. The smallest which has been measured is more than half a mile in diameter, and the largest about four miles. The phenomenon does not occur upon a small scale, but has a definite inferior limit to its magnitude. Let us seek an explanation of this limit.

The dome of strata which covers a laccolite has for its profile on every side a monoclinal curve. In Figure 52 the section of a dome exhibits a monoclinal flexure in _s a_ and again in _s b_; and the dome being approximately circular this flexure completely surrounds it. We may even describe or define the dome as a monoclinal flexure encircling a point or a space. Considering now that when the laccolite was injected the overlying strata were lifted, and that this disturbance was communicated upward to the then existing surface of the earth, we may properly speak of the lifted body of rock as a cylinder bounded on every side by a monoclinal flexure. Furthermore, since the monoclinal flexure is the structural equivalent of the fault[4], we may render our conception still simpler by replacing in imagination the encircling flexure by an encircling fault, and picturing to ourselves the uplifted rock mass as a simple cylinder, perfectly divided from the surrounding rock and slidden upward so as to project above the surface an amount equal to the depth of the laccolite.

Footnote 4:

Exploration of the Colorado, pp. 182–184. Explorations West of the 100th Meridian, Vol. III, p. 48. American Journal of Science, July, 1876, p. 21.

It is possible to give a mathematical expression to the force necessary to produce such a circular fault. Disregarding lithologic differences, the resistance to the rupture is measured by the area of the faulted surface, or what is the same thing, the area of the convex surface of the cylinder. Representing the resistance to be overcome by _r_, the height of the cylinder (equal to the depth of the cover of the laccolite) by _d_, and its circumference by _c_, we have

_r_ = _dcC_ (1),

in which _C_ is a function of the cohesion of the material and is constant.

The force by which the cylinder is lifted and by which it is assumed that the faulting is accomplished, is communicated through the molten lava of the forming laccolite. Being thus communicated it is applied equally to all parts of the base of the cylinder, and its efficient total is measured by the area of that base. A part of it is devoted to lifting the weight of the cylinder, and the remainder is devoted to the making of the fault. Each of these parts is proportioned, like the whole, to the area of the base of the cylinder, or to the area of the laccolite. Representing the portion applied to the faulting by _f_, and the area of the laccolite by _a_, we have

_f_ = _a C_{l}_

in which _C_{l}_, is a constant, and a function of the pressure under which the lava is injected.

Substituting for _a_ its equivalent, _c_^²⁄₄π

_f_ = _c^2 C_{l}_/4π

and substituting _C_{ll}_ for the constant term _C_{l}_/4π

_f_ = _c^2 C_{ll}_ (2)

Equation 1 gives an expression for the resistance which cohesion can oppose to the uplift of the cylinder. Equation 2 gives an expression for the force exerted by the fluid laccolite toward overcoming the resistance of cohesion. It is evident that for a given value of _d_ it is possible to assign a value of _C_ so large that _f_ will be greater than _r_, or so small that _f_ will be less than _r_. That is to say, at a given depth beneath the surface a laccolite of a certain circumference will be able to force upward the superjacent cylinder of rock, while a laccolite of a certain smaller circumference will be unable to lift its cover. Or in other words, there is a limit in size beneath which a laccolite cannot be formed.

When a lava forced upward through the strata reaches the level at which under the law of hydrostatic equilibrium it must stop, we may conceive that it expands along some plane of bedding in a thin sheet, until its horizontal extent becomes so great that it overcomes the resistance offered by the rigidity of its cover, and it begins to uplift it. The direction of least resistance is now upward, and the reservoir of lava increases in depth instead of width. The area of a laccolite thus tends to remain at its minimum limit, and may be regarded as more or less perfectly an index of that limit.

In equations 1 and 2, if _f_ = _r_, then

_c_^2 _C_{ll}_ = _d c C_

or

_c_ = _d_(_C_/_C_{ll}_) (3)

That is to say, if the force exerted by the lava is barely sufficient to overcome the resistance to uplift, then the circumference of the laccolite is proportional to the depth of its cover. Or in other words, the (linear) size of a laccolite is proportioned to its depth beneath the surface.

If now we return from the faulted cylinder which for simplicity’s sake has been hypothecated, to the actual cylinder which is surrounded by a flexure instead of a fault, can we retain our conclusions? With certain modifications I think we can. The strains developed in deformation by flexure are less easy of analysis than those which arise in faulting, but the two cases are in some degree analogous.

The expression (equation 2) for the force which the lava applies to deformation is unaffected by the manner in which the strata yield.

The expression (equation 1) for the resistance to deformation by faulting involves two terms, each in its simplest relations; the resistance varies directly as the circumference of the laccolite, and it varies directly as the depth of the cover. In order to pass to an expression for the resistance to deformation by flexure, only one of these terms need be changed. The resistance bears the same relation to the circumference of the laccolite; but it is no longer simply proportional to the depth of the cover. It varies more rapidly.

If the covering strata were all of a given thickness, were identical in kind, and were free to slide upon each other without friction, their total resistance to deformation would be equal to the resistance of a single stratum multiplied by the number of strata. But since they are not free to slide one upon another, they sustain each other, and the resistance offered by the combination is greater than that product.

I am led by the analogy of allied problems in mechanics to assume that the resistance of the body of strata varies with some power of its depth, but I am unable to say _what_ power. So far as I am aware, neither mathematical analysis nor experimentation has been directed to the problem in question. According to Rankine “the resistances of flexure of similar cross-sections [of elastic beams] are as their breadths and as the _squares_ of their depths” (“Applied Mechanics”, page 316), and it is possible that the same law applies to the resistances which continuous strata oppose to the uplifts of domes. But it appears more probable that the greater complexity of the strains developed in the formation of domes causes the depth to enter into the formula with a higher power than the second.

On the other hand, some allowance should be made for the fact that the elasticity of the resisting strata is imperfect.

If we call the power with which the depth enters the formula a, equation 1 becomes

_r_ = _d^a_ _c_ _C_{lll}_ (4).

and equation 3 becomes

_c_ = _d^a_ (_C_{lll}_/_C_{ll}_) (5).

It is probable that the true value of _a_ is not less than 2, nor more than 3.

Interpreting these equations in the same manner as those applying to deformation by faulting, we reach the following conclusions:

1st. At a given depth beneath the surface, lava injected under a given pressure cannot form a laccolite of less than a certain area. This may be called its _limital area_.

2d. The pressure of injection remaining constant, the limital area of a laccolite is a direct function of its depth beneath the surface. The limital area is greater when the depth is greater, and less when the depth is less.

3d. A laccolite of small volume will not exceed the limital area, but will grow by lifting its cover. If however the volume of intruded lava be great, its own weight becomes a factor in the equilibrium of forces and modifies the distribution of the pressures. As the rock bubble rises, the weight of the contained fluid is progressively subtracted from the pressure against its top, and this proceeds until the upward and lateral pressures become proportional to the resistances which severally oppose them. Further expansion is then both upward and outward.

4th. There is a limit to upward expansion, dependent on the fact that the pressure due to the combined weight of the laccolite and cover cannot exceed the pressure of the intrusive lava. Regarding the intrusive pressure as constant, it is divisible into three parts, of which one sustains the weight of the cover, also constant; another sustains the weight of the fluid laccolite, and is measured by its thickness or depth; and the third produces deformation. When the sum of the weights of the cover and laccolite equals the total pressure of the intrusive lava, uplift ceases, and the maximum depth or thickness is attained. We may call this the _limital thickness_. With regard to simple laccolites the limit is absolute, but it applies only to the distinct layers of those which are composite; for a composite laccolite, built by successive intrusions at wide intervals of time, may be relieved of part of its load by the erosion of the mound which its expansion causes at the surface of the land.

A laccolite formed beneath the bottom of a sea has a greater limital thickness than one formed beneath a land surface; for the superjacent water being displaced and thrust aside, is to that extent subtracted from the load to be lifted.

5th. The laccolite in its formation is constantly solving a problem of “least force”, and its form is the result. Below, above, and on all sides its expansion is resisted, and where the resistance is greatest its contour is least convex. The floor of its chamber is unyielding, and the bottom of the laccolite is flat. The roof and walls alike yield reluctantly to the pressure, but the weight of the lava diminishes its pressure on the roof. Hence the top of the laccolite becomes broadly convex, and its edges acutely. Local accidents excepted, the walls oppose an equal resistance on every side; and the base of the laccolite is rendered circular.

The second of the conclusions enunciated above is susceptible of test by observation. By selecting those laccolites of which the dimensions are known with the best degree of approximation, the following table has been formed:

│ Formations. │Titles of Laccolites.│Diameters│Means. │ │ │in miles.│

───────────┬───────────────────┬─────────────────────┬─────────┬───┬─── Upper Zone │Blue Gate Shale │Sentinel │ .7│ .7│1.2 ───────────┼───────────────────┼─────────────────────┼─────────┼───┼─── „ │Tununk Shale │Geikie │ .8│1.2│ „ „ │ „ │A │ .9│ „ │ „ „ │ „ │Marvine │ 1.0│ „ │ „ „ │ „ │Jukes │ 1.4│ „ │ „ „ │ „ │Peale │ 1.8│ „ │ „ ───────────┼───────────────────┼─────────────────────┼─────────┼───┼─── „ │Flaming Gorge Shale│Steward │ 1.0│1.4│ „ „ │ „ │B │ 1.1│ „ │ „ „ │ „ │Newberry │ 1.8│ „ │ „ „ │ „ │C │ 1.9│ „ │ „ ───────────┴───────────────────┼─────────────────────┼─────────┼───┼─── Lower Zone │Dana │ 2.0│ │2.6 „ │Greater Holmes │ 2.1│ │ „ „ │Lesser Holmes │ 2.1│ │ „ „ │Ellsworth │ 2.3│ │ „ „ │Pulpit │ 2.3│ │ „ „ │Maze │ 2.8│ │ „ „ │Crescent │ 3.6│ │ „ „ │Hillers │ 3.9│ │ „

There is no laccolite of the upper zone so large as the smallest in the lower zone; and the mean diameter of those in the lower zone is double the mean of those in the upper. The measurements do not give the diameters of limital areas, but it is presumable that the actual areas bear substantially the same relation to the limital in the two zones. If we select the smallest laccolites in each group as those most likely to express the limital areas, the result is practically the same.

│ Formations. │Diameters.│ Means. ───────────┼───────────────────┼──────────┼────┬──── Upper Zone │Tununk Shale │ .8│ .9 │1.0 „ │ „ │ .9│ „ │ „ „ │ „ │ 1.0│ „ │ „ ───────────┼───────────────────┼──────────┼────┼──── „ │Flaming Gorge Shale│ 1.0│1.05│ „ „ │ „ │ 1.1│ „ │ „ ───────────┴───────────────────┼──────────┼────┼──── Lower Zone │ 2.0│ │2.1 „ │ 2.1│ │ „ „ │ 2.1│ │ „

The mean for the lower zone is still double the mean for the upper.

The confirmation of the conclusion is as nearly perfect as could have been anticipated. There is no room to doubt that a relation exists between the diameters of laccolites and the depths of their intrusion.

Having determined by observation the mean size of the laccolites in the upper and lower zones, as well as the interval which separates the two zones, and knowing approximately the law which binds the size of the laccolite to its depth of intrusion, we can compute the depth of intrusion of each zone. Our result will doubtless have a large probable error, but it will not be entirely without value.

Let _x_ represent the thickness in feet of the original cover of the laccolites of the upper zone; and _x_ + 3300 the thickness of the cover of the laccolites of the lower zone. The mean circumference in feet of the upper laccolites is 1.2 π × 5280 = 6336 π. The mean circumference of the lower laccolites is 2.6 π × 5280 = 13728 π. Substituting these values in equation 5, we obtain

6336 π = _x^a_(_C_{lll}_/_C_{ll}_)

and

13728 π = (_x_ + 3300)_^a_(_C_{lll}_/_C_{ll}_)

Dividing the second equation by the first and reducing, 3300

_x_ = 3300/ª√(¹³⁷²⁸⁄₆₃₃₆ − 1) (6).

To obtain a minimum result, assume a = 2; then

_x_ = 3300/√(¹³⁷²⁸⁄₆₃₃₆ − 1) = 7000 feet,

and

_x_ + 3300 = 10300 feet.

The summit of the Masuk sandstone is 3,500 feet above the mean level of the upper laccolites; subtracting this from the value of _x_ gives 3,500 feet as the depth of Tertiary strata which overlay the Masuk beds during the epoch of laccolitic intrusion.

To obtain a maximum result, assume _a_ = 3; then

_x_ = 3300/∛(¹³⁷²⁸⁄₆₃₃₆ − 1) = 11200 feet,

_x_ + 3300 = 14500 feet,

and the result for the depth of the Tertiary strata is 7,700 feet.

I am far from attaching great weight to this speculation in regard to the original depths of the laccolite covers. It is always hazardous to attempt the quantitative discussion of geological problems, for the reason that the conditions are apt to be both complex and imperfectly known; and in this case an uncertainty attaches to the law of relation, as well as to the quantities to which it is applied. Nevertheless after making every allowance there remains a presumption that the cover of the laccolites included some thousands of feet of Tertiary sediments.

What evidence we have then, indicates that the epoch of laccolitic intrusion was after the accumulation of deep Tertiary deposits and before the subsequent degradation had made great progress—that it was at or near the close of the epoch of local Tertiary sedimentation.

If the reader would realize the relation between the eroded material and the surviving mountains, let him turn to the Frontispiece. A perspective view is there given of a tract ten miles square, with Mount Ellsworth in the center. It is represented as cut out from all surroundings by vertical planes which descend to the level of the ocean. The southern or nearer half of the block shows the present aspect of the country; the remote half shows the form it is supposed to have had if the uplift was completed before the erosion began, or what is the same thing, the form it would have, had there been no erosion. The difference between the two represents the total amount of the material that has been washed away since the completion of the Tertiary sediments.

Partly in review, let us now sketch the

HISTORY OF THE LACCOLITE.

When lavas forced upward from lower-lying reservoirs reach the zone in which there is the least hydrostatic resistance to their accumulation, they cease to rise. If this zone is at the top of the earth’s crust they build volcanoes; if it is beneath, they build laccolites. Light lavas are more apt to produce volcanoes; heavy, laccolites. The porphyritic trachytes of the Plateau Province produced laccolites.

The station of the laccolite being decided, the first step in its formation is the intrusion along a parting of strata, of a thin sheet of lava, which spreads until it has an area adequate, on the principle of the hydrostatic press, to the deformation of the covering strata. The spreading sheet always extends itself in the direction of least resistance, and if the resistances are equal on all sides, takes a circular form. So soon as the lava can uparch the strata it does so, and the sheet becomes a laccolite. With the continued addition of lava the laccolite grows in height and width, until finally the supply of material or the propelling force so far diminishes that the lava clogs by congelation in its conduit and the inflow stops. An irruption is then complete, and the progress of the laccolite is comparable with that of a volcano at the end of its first eruption. During the irruption and after its completion, there is an interchange of temperatures. The laccolite cools and solidifies; its walls are heated and metamorphosed. At the edges, where the surface of the laccolite is most convex, the heat is most rapidly dissipated, and its effect in metamorphism is least. A second irruption may take place either before or after the first is solidified. It may intrude above or it may intrude beneath it; and observation has not yet distinguished the one case from the other. In any case it carries forward the deformation of cover that was begun by the first, and combines with it in such way that the compound form is symmetric, and is substantially the same that would have been produced if the two irruptions were combined in one. Thus the laccolite grows by successive accretions until at length its cooled mass, heavier and stronger than the surrounding rocks, proves a sufficient obstacle to intrusion. The next irruption then avoids it, opens a new conduit, and builds a new laccolite at its side. By successive shiftings of the conduit a group of laccolites is formed, just as by the shifting of vents eruptive cones are grouped. Each laccolite is a subterranean volcano.

[Illustration:

FIG. 53.—Diagram to illustrate the relation of Dikes and Sheets to the Strains which are developed in the uplifting of laccolitic arches. ]

The strata above the laccolite are bent instead of broken, because their material is subjected to so great a pressure by superincumbent strata that it cannot hold an open fissure and is _quasi-plastic_. But although quasi-plastic it is none the less solid, and can be cracked open if the gap is instantaneously filled, the cracking and the filling being one event. This happens in the immediate walls of the laccolite, and they are injected by dikes and sheets of the lava. The directions of the cracks are normal to the directions of the extensive strains (strains tending to extend) where they occur. From the top of the laccolite dikes run upward into the roof, marking horizontal strains (_a a_). From the sides smaller vertical dikes run outward, marking horizontal, tangential strains. And parallel to the sides near the base of the laccolite, are numerous sheets, marking strains directed outward and upward (_c c_). These last especially serve to show that the rigidity of the strata is not abolished, although it is overpowered, by the pressure which warps them.

Here we are brought face to face with a great fact of dynamic geology which though well known is too often ignored. The solid crust of the earth, and the solid earth if it be solid, are as plastic _in great masses_ as wax is in small. Solidity is not absolute but relative. It is only a low grade of plasticity. The rigidity or strength of a body is measured by the square of its linear dimensions, while its weight is measured by the cube. Hence with increase in magnitude, the weight increases more rapidly than the strength; and no very large body is strong enough to withstand the pressure of its own weight. However solid it may be, it must succumb and be flattened. When we speak of rock masses which are measured by feet, we may regard them as solid; but when we consider masses which are measured by miles, we should regard them as plastic.

The same principle is illustrated by the limital area of laccolites. A small laccolite cannot lift its small cover, but a large laccolite can lift its correspondingly large cover. The strength or rigidity which resists deformation is overcome by magnitude.

_Laccolites of Other Regions._—In many lands geologists have observed intrusive rocks occurring in great bodies, but I am not aware that such a system as that of the Henry Mountains has ever been described. Doubtless all such bodies are laccolitic, but the combination of conditions which this field presents can rarely be repeated. In the first place the strata which here contain the laccolites lay level. They had suffered no displacement before the epoch of irruption, and they have suffered none since. The laccolitic phenomena stand by themselves, with nothing to mar their symmetry or complicate their study. In the next place the laccolites are here assembled in such number and with such variety of size, form, and horizon that there is little danger of mistaking accidental features for essential. Again, the region having been recently elevated is the scene of rapid degradation. Waterways are deeply corraded, slopes are steep, and escarpments abound. And finally the climate is so arid that vegetation is exceedingly scant. The rocks are for the most part bare and their examination is unobstructed.

If the conditions of erosion and climate had been unfavorable in the Henry Mountains, they could not have yielded the key to the laccolitic structure; but the key once found, it is to be anticipated that the structure will be recognized in other laccolites of which the exposures are less perfect.

If the strata had experienced anterior displacements so as to be inclined, folded, and faulted, a symmetrical growth of laccolites would have been impossible, and the mountains would not have yielded a knowledge of the type form. But the type form being known, it is to be anticipated that in disturbed regions aberrant forms will be recognized and referred to the type.

_Possible Analogues of the Laccolite._—All the arches of the Henry Mountains have been ascribed to laccolites, whether their nuclei were visible or concealed, and the evidence upon which the latter were included appears to admit of no controversy. The question arises whether the great flexures of the Plateau region may not be allied in structure. The volcano having its homologue in the laccolite, may not broad lava fields have their homologues beneath displacements of the Kaibab type?

The idea is naturally attractive to one who has made a special study of laccolites, but it is hardly tenable. There are indeed many points of resemblance between such flexures as the Waterpocket, and the uplifts of the Henry Mountains; but the points of contrast are equally conspicuous, and seem to mark a radical difference.

There is a certain symmetry of form which is characteristic of the laccolitic arches, but which is rarely seen in the great flexures. And there is a linear element which is characteristic of the latter, but not of the former. The great flexures always have direction or trend, and often exhibit parallelism; the laccolitic arches betray no trend either individually or collectively.

These features are well shown in Plate II, where the Waterpocket flexure is contrasted with the Henry Mountain arches.