CHAPTER II.

THE STRUCTURE OF THE HENRY MOUNTAINS.

The mountains stand within the province of the great flexures, but are independent of them. Fifteen miles to the westward runs the Waterpocket flexure. Thirty miles to the north is the San Rafael fold. At the east the strata rise toward a great uplift, of which the full form is unknown. But where the Henry Mountains stand the rocks are unaffected by these disturbances. They have a uniform dip of about 45′ to the northwest, and form a perfect datum-plane from which to measure the magnitude of the displacements which have given rise to the mountains.

The mountains are composed of a large number of parts which are in a certain degree individual and homologous. By the generalization of the characters of those parts a conception has been obtained of a _type structure_ to which the entire series of phenomena has been referred.

In laying the material before the reader, the following plan will be followed:

First. The type of structure will be briefly set forth.

Second. The phenomena by which the type is at once demonstrated and illustrated will be described in detail.

Third. The type of structure will be discussed.

If the structure of the mountains be as novel to the reader as it was to the writer, and if it be as strongly opposed to his preconception of the manner in which igneous mountains are constituted, he may well question the conclusions in regard to it while they are unsustained by proof. I can only beg him to suspend his judgment until the whole case shall have been presented. On some accounts it would have been well to follow in writing the order of investigation, and develop the general plan of structure as it was developed in the field, by the addition here of one element and there of another; or at least to assemble the facts before announcing my deductions. But such a course would be at the expense of an important element of convenience and brevity. As will appear in the sequel, the preliminary explanation of the type structure furnishes a complement of categories and terms by the aid of which the description of the details of observation, essentially tedious, is greatly abbreviated.

[Illustration:

FIG. 7.—Ideal Cross-section of a Mountain of Eruption. ]

[Illustration:

FIG. 8.—Ideal Cross-section of a Laccolite, showing the typical form and the arching of the overlying strata. ]

It is usual for igneous rocks to ascend to the surface of the earth, and there issue forth and build up mountains or hills by successive eruptions. The molten matter starting from some region of unknown depth passes through all superincumbent rock-beds, and piles itself up on the uppermost bed. The lava of the Henry Mountains behaved differently. Instead of rising through all the beds of the earth’s crust, it stopped at a lower horizon, insinuated itself between two strata, and opened for itself a chamber by lifting all the superior beds. In this chamber it congealed, forming a massive body of trap. For this body the name _laccolite_ (λάκκος, _cistern_, and λίθος, _stone_) will be used. Figure 7 and Figure 8 are ideal sections of a mountain of eruption and of a laccolite.

The laccolite is the chief element of the type of structure exemplified in the Henry Mountains.

It is evident that the intrusion of a laccolite will produce upon the surface as great a hill as the extrusion of the same quantity of matter, the mass which is carried above the original surface being precisely equivalent to that which is displaced by the laccolite; and it is further evident that where the superior rock is horizontally stratified every stratum above the laccolite will be uplifted, and, unless it is fractured, will be upbent, and will portray, more or less faithfully, by its curvature, the form of the body it covers.

Associated with the laccolites of the Henry Mountains are _sheets_ and _dikes_.

The term _sheet_ will be applied in this report to broad, thin, stratified bodies of trap, which have been intruded along the partings between sedimentary strata, and conform with the inclosing strata in dip. _Dikes_ differ from sheets in that they intersect the sedimentary strata at greater or less angles, occupying fissures produced by the rupture of the strata.

The logical distinction between dike and sheet is complete, but in nature it not unfrequently happens that the same body of trap is a sheet in one place and a dike in another. Between the sheet and the laccolite there is a complete gradation. The laccolite is a greatly thickened sheet, and the sheet is a broad, thin, attenuated laccolite.

[Illustration:

FIG. 9.—Ideal Cross-section of a Laccolite, with accompanying Sheets and Dikes. ]

In the district under consideration the laccolite is usually, perhaps always, accompanied by dikes and sheets (see Figure 9). There are sheets beneath laccolites and sheets above them. The superior sheets have never been observed to extend beyond the curved portion of the superior strata. Dikes rise from the upper surfaces of the laccolites. They are largest and most numerous about the center, but, like the superior sheets, they often extend nearly to the limit of the flexure of the uplifted strata. The larger often radiate from the center outward, but there is no constancy of arrangement. Where they are numerous they reticulate.

In the accompanying diagrams dikes are represented beneath as well as above the laccolites. These are purely hypothetical, since they have not been seen. In a general way, the molten rock must have come from below, but the channel by which it rose has in no instance been determined by observation.

The horizontal distribution of the laccolites is as irregular as the arrangement of volcanic vents. They lie in clusters, and each cluster is marked by a mountain. In Mount Ellen there are perhaps thirty laccolites. In Mount Holmes there are two; and in Mount Ellsworth one. Mount Pennell and Mount Hillers each have one large and several smaller ones.

[Illustration:

FIG. 10.—Ideal Cross-section of Grouped Laccolites. ]

Their vertical distribution likewise is irregular. Some have intruded themselves between Cretaceous strata, others between Jura-Triassic, and others between Carboniferous. From the highest to the lowest the range is not less than 4,000 feet. Those which are above not unfrequently overlap those which lie below, as represented in the ideal section, Figure 10.

The erosion of the mountains has given the utmost variety of exposure to the laccolites. In one place are seen only arching strata; in another, arching strata crossed by a few dikes; in another, arching strata filled with a network of dikes and sheets. Elsewhere a portion of the laccolite itself is bared, or one side is removed so as to exhibit a natural section. Here the sedimentary cover has all been removed, and the laccolite stands free, with its original form; there the hard trachyte itself has been attacked by the elements and its form is changed. Somewhere, perhaps, the laccolite has been destroyed and only a dike remains to mark the fissure through which it was injected.