CHAPTER XXVIII

.

VOLCANIC ROCKS.

External Form, Structure, and Origin of Volcanic Mountains. — Cones and Craters. — Hypothesis of “Elevation Craters” considered. — Trap Rocks. — Name whence derived. — Minerals most abundant in Volcanic Rocks. — Table of the Analysis of Minerals in the Volcanic and Hypogene Rocks. — Similar Minerals in Meteorites. — Theory of Isomorphism. — Basaltic Rocks. — Trachytic Rocks. — Special Forms of Structure. — The columnar and globular Forms. — Trap Dikes and Veins. — Alteration of Rocks by volcanic Dikes. — Conversion of Chalk into Marble. — Intrusion of Trap between Strata. — Relation of trappean Rocks to the Products of active Volcanoes.

The aqueous or fossiliferous rocks having now been described, we have next to examine those which may be called volcanic, in the most extended sense of that term. In the diagram (Fig. 584) suppose _a, a_ to represent the crystalline formations, such as the granitic and metamorphic; _b, b_ the fossiliferous strata; and _c, c_ the volcanic rocks. These last are sometimes found, as was explained in the first chapter, breaking through _a_ and _b,_ sometimes overlying both, and occasionally alternating with the strata _b, b._

Fig. 584: a. Hypogene formations, stratified and unstratified. b. Aqueous formations. c. Volcanic rocks.

External Form, Structure, and Origin of Volcanic Mountains.—The origin of volcanic cones with crater-shaped summits has been explained in the “Principles of Geology” (Chapters 23 to 27), where Vesuvius, Etna, Santorin, and Barren Island are described. The more ancient portions of those mountains or islands, formed long before the times of history, exhibit the same external features and internal structure which belong to most of the extinct volcanoes of still higher antiquity; and these last have evidently been due to a complicated series of operations, varied in kind according to circumstances; as, for example, whether the accumulation took place above or below the level of the sea, whether the lava issued from one or several contiguous vents, and, lastly, whether the rocks reduced to fusion in the subterranean regions happened to have contained more or less silica, potash, soda, lime, iron, and other ingredients. We are best acquainted with the effects of eruptions above water, or those called subÆrial or supramarine; yet the products even of these are arranged in so many ways that their interpretation has given rise to a variety of contradictory opinions, some of which will have to be considered in this chapter.

Fig. 585: Part of the chain of extinct volcanoes called the Monts Dome, Aurvergne.

_Cones and Craters._—In regions where the eruption of volcanic matter has taken place in the open air, and where the surface has never since been subjected to great aqueous denudation, cones and craters constitute the most striking peculiarity of this class of formations. Many hundreds of these cones are seen in central France, in the ancient provinces of Auvergne, Velay, and Vivarais, where they observe, for the most part, a linear arrangement, and form chains of hills. Although none of the eruptions have happened within the historical era, the streams of lava may still be traced distinctly descending from many of the craters, and following the lowest levels of the existing valleys. The origin of the cone and crater-shaped hill is well understood, the growth of many having been watched during volcanic eruptions. A chasm or fissure first opens in the earth, from which great volumes of steam are evolved. The explosions are so violent as to hurl up into the air fragments of broken stone, parts of which are shivered into minute atoms. At the same time melted stone or _lava_ usually ascends through the chimney or vent by which the gases make their escape. Although extremely heavy, this lava is forced up by the expansive power of entangled gaseous fluids, chiefly steam or aqueous vapour, exactly in the same manner as water is made to boil over the edge of a vessel when steam has been generated at the bottom by heat. Large quantities of the lava are also shot up into the air, where it separates into fragments, and acquires a spongy texture by the sudden enlargement of the included gases, and thus forms _scoriæ,_ other portions being reduced to an impalpable powder or dust. The showering down of the various ejected materials round the orifice of eruption gives rise to a conical mound, in which the successive envelopes of sand and scoriæ form layers, dipping on all sides from a central axis. In the mean time a hollow, called a _ crater,_ has been kept open in the middle of the mound by the continued passage upward of steam and other gaseous fluids. The lava sometimes flows over the edge of the crater, and thus thickens and strengthens the sides of the cone; but sometimes it breaks down the cone on one side (see Fig. 585), and often it flows out from a fissure at the base of the hill, or at some distance from its base.

Some geologists had erroneously supposed, from observations made on recent cones of eruption, that lava which consolidates on steep slopes is always of a scoriaceous or vesicular structure, and never of that compact texture which we find in those rocks which are usually termed “trappean.” Misled by this theory, they have gone so far as to believe that if melted matter has originally descended a slope at an angle exceeding four or five degrees, it never, on cooling, acquires a stony compact texture. Consequently, whenever they found in a volcanic mountain sheets of stony materials inclined at angles of from 5° to 20° or even more than 30°, they thought themselves warranted in assuming that such rocks had been originally horizontal, or very slightly inclined, and had acquired their high inclination by subsequent upheaval. To such dome-shaped mountains with a cavity in the middle, and with the inclined beds having what was called a quâquâversal dip or a slope outward on all sides, they gave the name of “Elevation craters.”

As the late Leopold Von Buch, the author of this theory, had selected the Isle of Palma, one of the Canaries, as a typical illustration of this form of volcanic mountain, I visited that island in 1854, in company with my friend Mr. Hartung, and I satisfied myself that it owes its origin to a series of eruptions of the same nature as those which formed the minor cones, already alluded to. In some of the more ancient or Miocene volcanic mountains, such as Mont Dor and Cantal in central France, the mode of origin by upheaval as above described is attributed to those dome-shaped masses, whether they possess or not a great central cavity, as in Palma. Where this cavity is present, it has probably been due to one or more great explosions similar to that which destroyed a great part of ancient Vesuvius in the time of Pliny. Similar paroxysmal catastrophes have caused in historical times the truncation on a grand scale of some large cones in Java and elsewhere.[1]

Among the objections which may be considered as fatal to Von Buch’s doctrine of upheaval in these cases, I may state that a series of volcanic formations extending over an area six or seven miles in its shortest diameter, as in Palma, could not be accumulated in the form of lavas, tuffs, and volcanic breccias or agglomerates without producing a mountain as lofty as that which they now constitute. But assuming that they were first horizontal, and then lifted up by a force acting most powerfully in the centre and tilting the beds on all sides, a central crater having been formed by explosion or by a chasm opening in the middle, where the continuity of the rocks was interrupted, we should have a right to expect that the chief ravines or valleys would open towards the central cavity, instead of which the rim of the great crater in Palma and other similar ancient volcanoes is entire for more than three parts of the whole circumference.

If dikes are seen in the precipices surrounding such craters or central cavities, they certainly imply rents which were filled up with liquid matter. But none of the dislocations producing such rents can have belonged to the supposed period of terminal and paroxysmal upheaval, for had a great central crater been already formed before they originated, or at the time when they took place, the melted matter, instead of filling the narrow vents, would have flowed down into the bottom of the cavity, and would have obliterated it to a certain extent. Making due allowance for the quantity of matter removed by subaërial denudation in volcanic mountains of high antiquity, and for the grand explosions which are known to have caused truncation in

## active volcanoes, there is no reason for calling in the violent

hypothesis of elevation craters to explain the structure of such mountains as Teneriffe, the Grand Canary, Palma, or those of central France, Etna, or Vesuvius, all of which I have examined. With regard to Etna, I have shown, from observations made by me in 1857, that modern lavas, several of them of known date, have formed continuous beds of compact stone even on slopes of 15, 36, and 38 degrees, and, in the case of the lava of 1852, more than 40 degrees. The thickness of these tabular layers varies from 1½ foot to 26 feet. And their planes of stratification are parallel to those of the overlying and underlying scoriæ which form part of the same currents.[2]

Nomenclature of Trappean Rocks.—When geologists first began to examine attentively the structure of the northern and western parts of Europe, they were almost entirely ignorant of the phenomena of existing volcanoes. They found certain rocks, for the most part without stratification, and of a peculiar mineral composition, to which they gave different names, such as basalt, greenstone, porphyry, trap tuff, and amygdaloid. All these, which were recognised as belonging to one family, were called “trap” by Bergmann, from _trappa,_ Swedish for a flight of steps—a name since adopted very generally into the nomenclature of the science; for it was observed that many rocks of this class occurred in great tabular masses of unequal extent, so as to form a succession of terraces or steps. It was also felt that some general term was indispensable, because these rocks, although very diversified in form and composition, evidently belonged to one group, distinguishable from the Plutonic as well as from the non-volcanic fossiliferous rocks.

By degrees familiarity with the products of active volcanoes convinced geologists more and more that they were identical with the trappean rocks. In every stream of modern lava there is some variation in character and composition, and even where no important difference can be recognised in the proportions of silica, alumina, lime, potash, iron, and other elementary materials, the resulting materials are often not the same, for reasons which we are as yet unable to explain. The difference also of the lavas poured out from the same mountain at two distinct periods, especially in the quantity of silica which they contain, is often so great as to give rise to rocks which are regarded as forming distinct families, although there may be every intermediate gradation between the two extremes, and although some rocks, forming a transition from the one class to the other, may often be so abundant as to demand special names. These species might be multiplied indefinitely, and I can only afford space to name a few of the principal ones, about the composition and aspect of which there is the least discordance of opinion.

Minerals most abundant in Volcanic Rocks.—The minerals which form the chief constituents of these igneous rocks are few in number. Next to quartz, which is nearly pure silica or silicic acid, the most important are those silicates commonly classed under the several heads of feldspar, mica, hornblende or augite, and olivine. In Table 28.1, in drawing up which I have received the able assistance of Mr. David Forbes, the chemical analysis of these minerals and their varieties is shown, and he has added the specific gravity of the different mineral species, the geological application of which in determining the rocks formed by these minerals will be explained in the sequel (p.504).

_Analysis of Minerals most abundant in the Volcanic and Hypogene Rocks._

THE QUARTZ GROUP QUARTZ 100·0 2·6 Silica Specific gravity TRIDYMITE 100·0 2·3 Silica Specific gravity THE FELDSPAR GROUP ORTHOCLASE. —— Carisbad, in granite (bulk) 65·23 16·26 0·27 nil trace nil 14·66 1·45 nil 2·55 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity —— Sanadine, Drachenfels in trachyte (Rammelsberg) 65·87 18·53 nil nil 0·95 0·30 10·32 3·49 W. 0·44 2·55 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity ALBITE. —— Arendal, in granite (G. Rose) 68·46 19·30 nil 0·28 0·68 nil nil 11·27 nil 2·61 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity OLIGOCLASE. —— Ytterby, in granite (Berzelius) 61·55 23·80 nil nil 3·18 0·80 0·38 9·67 nil 2·65 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity —— Teneriffe, in trachyte (Deville) 61·55 22·03 nil nil 2·81 0·47 3·44 7·74 nil 2·59 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity LABRADORITE. —— Hitteroe, in Labrador-rock (Waage) 51·39 29·42 2·90 nil 9·44 0·37 1·10 5·03 W. 0·71 2·72 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity —— Iceland, in volcanic (Damour) 52·17 29·22 1·90 nil 13·11 nil nil 3·40 nil 2·71 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity ANORTHITE. —— Harzburg, in diorite (Streng) 45·37 34·81 0·59 nil 16·52 0·83 0·40 1·45 W. 0·87 2·74 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity —— Hecla, in volcanic (Waltershausen) 45·14 32·10 2·03 0·78 18·32 nil 0·22 1·06 nil 2·74 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity LEUCITE. —— Vesuvius, 1811, in lava (Rammelsberg) 56·10 23·22 nil nil nil nil 20·59 0·57 nil 2·48 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity NEPHELINE. —— Miask, in Miascite (Scheerer) 44·30 33·25 0·82 nil 0·32 0·07 5·82 16·02 nil 2·59 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity —— Vesuvius, in volcanic (Arfvedson) 44·11 33·73 nil nil nil nil nil 20·46 W. 0·62 2·60 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity THE MICA GROUP MUSCOVITE. —— Finland, in grante (Rose) 46·36 36·80 4·53 nil nil nil 9·22 nil F. 0·67 W. 1·84 2·90 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents

Specific gravity LEPIDOLITE. —— Cornwall, in granite (Regnault) 52·40 26·80 nil 1·50 nil nil 9·14 nil F. 4·18 Li. 4·85 2·90 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents

Specific gravity BIOTITE. —— Bodennais (V. Kobel> 40·86 15·13 13·00 nil nil 22·00 8·83 nil W. 0·44 2·70 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity —— Vesuvius, in volcanic (Chodnef) 40·91 17·71 11·02 nil 0·30 19·04 9·96 nil nil 2·75 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity PHLOGOPITE. —— New York, in metamorphic limestone (Rammelsberg) 41·96 13·47 nil 2·67 0·34 27·12 9·37 nil F. 2·93 W. 0·60 2·81 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents

Specific gravity MARGARITE. —— Nexos (Smith) 30·02 49·52 1·65 nil 10·82 0·48 1·25

W. 5·55 2·99 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia =Potash =Soda Other constituents Specific gravity RAPIDOLITE. —— Pyrenees (Delesse) 32·10 18·50 nil 0·06 nil 36·70 nil nil W. 12·10 2·61 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity TALC. —— Zillerthal (Delesse) 63·00 nil nil trace nil 33·60 nil nil W. 3·10 2·78 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity THE AMPHIBOLE AND PYROXENE GROUP TREMOLITE. —— St. Gothard (Rammelsbeg) 58·55 nil nil nil 13·90 26·63 nil nil F.W. 0·34 2·93 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity ACTINOLITE. —— Arendal, in granite (Rammelsberg) 56·77 0·97 nil 5·88 13·56 21·48 nil nil W. 2·20 3·02 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity HORNBLENDE. —— Faymont, in diorite (Deville) 41·99 11·66 nil 22·22 9·55 12·59 nil 1·02 W. 1·47 3·20 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity —— Etna, in volcanic (Waltershausen) 40·91 13·68 nil 17·49 13·44 13·19 nil nil W. 0·85 3·01 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity URALITE. —— Ural, (Rammelsberg) 50·75 5·65 nil 17·27 11·59 12·28 nil nil W. 1·80 3·14 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity AUGITE. —— Bohemia, in dolerite (Rammelsberg) 51·12 3·38 0·95 8·08 23·54 12·82 nil nil nil 3·35 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity —— Vesuvius, in lava of 1858 (Rammelsberg) 49·61 4·42 nil 9·08 22·83 14·22 nil nil nil 3·25 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity DIALLAGE. —— Harz, in Gabbro (Rammelsberg) 52·00 3·10 nil 9·36 16·29 18·51 nil nil W. 1·10 3·23 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity HYPERSTHENE. —— Labrador, in Labrador-Rock (Damour) 51·36 0·37 nil 22·59 3·09 21·31 nil nil nil 3·39 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity THE OLIVINE GROUP BRONZITE. —— Greenland (V. Kobell) 58·00 1·33 11·14 nil nil 29·66 nil nil nil 3·20 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity OLIVINE. —— Carlsbad, in basalt (Rammelsberg) 39·34 nil nil 14·85 nil 45·81 nil nil nil 3·40 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity —— Mount Somma, in volcanic (Walmstedt) 10·08 0·18 nil 15·74 nil 44·22 nil nil nil 3·33 Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity

In the “Other constituents” the following signs are used: F=Fluorine, Li=Lithia, W=Loss on igniting the mineral, in most instances only Water.

From the table above it will be observed that many minerals are omitted which, even if they are of common occurrence, are more to be regarded as accessory than as essential components of the rocks in which they are found.[3] Such are, for example, Garnet, Epidote, Tourmaline, Idocrase, Andalusite, Scapolite, the various Zeolites, and several other silicates of somewhat rarer occurrence. Magnetite, Titanoferrite, and Iron-pyrites also occur as normal constituents of various igneous rocks, although in very small amount, as also Apatite, or phosphate of lime. The other salts of lime, including its carbonate or calcite, although often met with, are invariably products of secondary chemical

## action.

The Zeolites, above mentioned, so named from the manner in which they froth up under the blow-pipe and melt into a glass, differ in their chemical composition from all the other mineral constituents of volcanic rocks, since they are hydrated silicates containing from 10 to 25 per cent of water. They abound in some trappean rocks and ancient lavas, where they fill up vesicular cavities and interstices in the substance of the rocks, but are rarely found in any quantity in recent lavas; in most cases they are to be regarded as secondary products formed by the action of water on the other constituents of the rocks. Among them the species Analcime, Stilbite, Natrolite, and Chabazite may be mentioned as of most common occurrence.

Quartz Group.—The microscope has shown that pure quartz is oftener present in lavas than was formerly supposed. It had been argued that the quartz in granite having a specific gravity of 2·6, was not of purely igneous origin, because the silica resulting from fusion in the laboratory has only a specific gravity of 2·3. But Mr. David Forbes has ascertained that the free quartz in trachytes, which are known to have flowed as lava, has the same specific gravity as the ordinary quartz of granite; and the recent researches of Von Rath and others prove that the mineral Tridymite, which is crystallised silica of specific gravity 2·3 (see Table, p. 499), is of common occurrence in the volcanic rocks of Mexico, Auvergne, the Rhine, and elsewhere, although hitherto entirely overlooked.

Feldspar Group.—In the Feldspar group (Table, p. 499) the five mineral species most commonly met with as rock constituents are: 1. Orthoclase, often called common or potash-feldspar. 2. Albite, or soda-feldspar, a mineral which plays a more subordinate part than was formerly supposed, this name having been given to much which has since been proved to be Oligoclase. 3. Oligoclase, or soda-lime feldspar, in which soda is present in much larger proportion than lime, and of which mineral andesite are andesine, is considered to be a variety. 4. Labradorite, or lime-soda-feldspar, in which the proportions of lime and soda are the reverse to what they are in Oligoclase. 5. Anorthite or lime-feldspar. The two latter feldspars are rarely if ever found to enter into the composition of rocks containing quartz.

In employing such terms as potash-feldspar, etc., it must, however, always be borne in mind that it is only intended to direct attention to the predominant alkali or alkaline earth in the mineral, not to assert the absence of the others, which in most cases will be found to be present in minor quantity. Thus potash-feldspar (orthoclase) almost always contains a little soda, and often traces of lime or magnesia; and in like manner with the others. The terms “glassy” and “compact” feldspars only refer to structure, and not to species or composition; the student should be prepared to meet with any of the above feldspars in either of these conditions: the glassy state being apparently due to quick cooling, and the compact to conditions unfavourable to crystallisation; the so-called “compact feldspar” is also very commonly found to be an admixture of more than one feldspar species, and frequently also contains quartz and other extraneous mineral matter only to be detected by the microscope.

Feldspars when arranged according to their system of crystallisation are _monoclinic,_ having one axis obliquely inclined; or _triclinic,_ having the three axes all obliquely inclined to each other. If arranged with reference to their cleavage they are _orthoclastic,_ the fracture taking place always at a right angle; or _plagioclastic,_ in which the cleavages are oblique to one another. Orthoclase is orthoclastic and monoclinic; all the other feldspars are plagioclastic and triclinic.

_Minerals in Meteorites._—That variety of the Feldspar Group which is called Anorthite has been shown by Rammelsberg to occur in a meteoric stone, and his analysis proves it to be almost identical in its chemical proportions to the same mineral in the lavas of modern volcanoes. So also Bronzite (Enstatite) and Olivine have been met with in meteorites shown by analysis to come remarkably near to these minerals in ordinary rocks.

Mica Group.—With regard to the micas, the four principal species (Table, p. 499) all contain potash in nearly the same proportion, but differ greatly in the proportion and nature of their other ingredients. Muscovite is often called common or potash mica; Lepidolite is characterised by containing lithia in addition; Biotite contains a large amount of magnesia and oxide of iron; whilst Phlogopite contains still more of the former substance. In rocks containing quartz, muscovite or lepidolite are most common. The mica in recent volcanic rocks, gabbros, and diorites is usually Biotite, while that so common in metamorphic limestones is usually, if not always, Phlogopite.

Amphibole and Pyroxene Group.—The minerals included in the table under the Amphibole and Pyroxene Group differ somewhat in their crystallisation form, though they all belong to the monoclinic system. Amphibole is a general name for all the different varieties of Hornblende, Actinolite, Tremolite, etc., while Pyroxene includes Augite, Diallage, Malacolite, Sahlite, etc. The two divisions are so much allied in chemical composition and crystallographic characters, and blend so completely one into the other in Uralite (see page 499), that it is perhaps best to unite them in one group.

Theory of Isomorphism.—The history of the changes of opinion on this point is curious and instructive. Werner first distinguished augite from hornblende; and his proposal to separate them obtained afterwards the sanction of Haüy, Mohs, and other celebrated mineralogists. It was agreed that the form of the crystals of the two species was different, and also their structure, as shown by _cleavage_—that is to say, by breaking or cleaving the mineral with a chisel, or a blow of the hammer, in the direction in which it yields most readily. It was also found by analysis that augite usually contained more lime, less alumina, and no fluoric acid; which last, though not always found in hornblende, often enters into its composition in minute quantity. In addition to these characters, it was remarked as a geological fact, that augite and hornblende are very rarely associated together in the same rock. It was also remarked that in the crystalline slags of furnaces augitic forms were frequent, the hornblendic entirely absent; hence it was conjectured that hornblende might be the result of slow, and augite of rapid cooling. This view was confirmed by the fact that Mitscherlich and Berthier were able to make augite artificially, but could never succeed in forming hornblende. Lastly, Gustavus Rose fused a mass of hornblende in a porcelain furnace, and found that it did not, on cooling, assume its previous shape, but invariably took that of augite. The same mineralogist observed certain crystals called Uralite (see Table, p. 499) in rocks from Siberia, which possessed the cleavage and chemical composition of hornblende, while they had the external form of augite.

If, from these data, it is inferred that the same substance may assume the crystalline forms of hornblende or augite indifferently, according to the more or less rapid cooling of the melted mass, it is nevertheless certain that the variety commonly called augite, and recognised by a peculiar crystalline form, has usually more lime in it, and less alumina, than that called hornblende, although the quantities of these elements do not seem to be always the same. Unquestionably the facts and experiments above mentioned show the very near affinity of hornblende and augite; but even the convertibility of one into the other, by melting and recrystallising, does not perhaps demonstrate their absolute identity. For there is often some portion of the materials in a crystal which are not in perfect chemical combination with the rest. Carbonate of lime, for example, sometimes carries with it a considerable quantity of silex into its own form of crystal, the silex being mechanically mixed as sand, and yet not preventing the carbonate of lime from assuming the form proper to it. This is an extreme case, but in many others some one or more of the ingredients in a crystal may be excluded from perfect chemical union; and after fusion, when the mass recrystallises, the same elements may combine perfectly or in new proportions, and thus a new mineral may be produced. Or some one of the gaseous elements of the atmosphere, the oxygen for example, may, when the melted matter reconsolidates, combine with some one of the component elements.

The different quantity of the impurities or the refuse above alluded to, which may occur in all but the most transparent and perfect crystals, may partly explain the discordant results at which experienced chemists have arrived in their analysis of the same mineral. For the reader will often find that crystals of a mineral determined to be the same by physical characters, crystalline form, and optical properties, have been declared by skilful analysers to be composed of distinct elements. This disagreement seemed at first subversive of the atomic theory, or the doctrine that there is a fixed and constant relation between the crystalline form and structure of a mineral and its chemical composition. The apparent anomaly, however, which threatened to throw the whole science of mineralogy into confusion, was reconciled to fixed principles by the discoveries of Professor Mitscherlich at Berlin, who ascertained that the composition of the minerals which had appeared so variable was governed by a general law, to which he gave the name of _isomorphism_ (from _ isos,_ equal, and _morphe,_ form). According to this law, the ingredients of a given species of mineral are not absolutely fixed as to their kind and quality; but one ingredient may be replaced by an equivalent portion of some analogous ingredient. Thus, in augite, the lime may be in part replaced by portions of protoxide of iron, or of manganese, while the form of the crystal, and the angle of its cleavage planes, remain the same. These vicarious substitutions, however, of particular elements cannot exceed certain defined limits.

Basaltic Rocks.—The two principal families of trappean or volcanic rocks are the basalts and the trachytes, which differ chiefly from each other in the quantity of silica which they contain. The basaltic rocks are comparatively poor in silica, containing less than 50 per cent of that mineral, and none in a pure state or as free quartz, apart from the rest of the matrix. They contain a larger proportion of lime and magnesia than the trachytes, so that they are heavier, independently of the frequent presence of the oxides of iron which in some cases forms more than a fourth part of the whole mass. Abich has, therefore, proposed that we should weigh these rocks, in order to appreciate their composition in cases where it is impossible to separate their component minerals. Thus, basalt from Staffa, containing 47·80 per cent of silica, has a specific gravity of 2·95; whereas trachyte, which has 66 per cent of silica, has a specific gravity of only 2·68; trachytic porphyry, containing 69 per cent of silica, a specific gravity of only 2·58. If we then take a rock of intermediate composition, such as that prevailing in the Peak of Teneriffe, which Abich calls Trachyte-dolerite, its proportion of silica being intermediate, or 58 per cent, it weighs 2·78, or more than trachyte, and less than basalt.[4]

_Basalt._—The different varieties of this rock are distinguished by the names of basalts, anamezites, and dolerites, names which, however, only denote differences in texture without implying any difference in mineral or chemical composition: the term _Basalt_ being used only when the rock is compact, amorphous, and often semi-vitreous in texture, and when it breaks with a perfect conchoidal fracture; when, however, it is uniformly crystalline in appearance, yet very close-grained, the name _ Anamesite_ (from _anamesos,_ intermediate) is employed, but if the rock be so coarsely crystallised that its different mineral constituents can be easily recognised by the eye, it is called _ Dolerite_ (from _doleros,_ deceitful), in allusion to the difficulty of distinguishing it from some of the rocks known as Plutonic.

_Melaphyre_ is often quite undistinguishable in external appearance from basalt, for although rarely so heavy, dark-coloured, or compact, it may present at times all these varieties of texture. Both these rocks are composed of triclinic feldspar and augite with more or less olivine, magnetic or titaniferous oxide of iron, and usually a little nepheline, leucite, and apatite; basalt usually contains considerably more olivine than melaphyre, but chemically they are closely allied, although the melaphyres usually contain more silica and alumina, with less oxides of iron, lime, and magnesia, than the basalts. The Rowley Hills in Staffordshire, commonly known as Rowley Ragstone, are melaphyre.

_Greenstone._—This name has usually been extended to all granular mixtures, whether of hornblende and feldspar, or of augite and feldspar. The term _diorite_ has been applied exclusively to compounds of hornblende and triclinic feldspar. _ Labrador-rock_ is a term used for a compound of labradorite or labrador-feldspar and hypersthene; when the hypersthene predominates it is sometimes known under the name of _ Hypersthene-rock._ _Gabbro_ and _Diabase_ are rocks mainly composed of triclinic feldspars and diallage. All these rocks become sometimes very crystalline, and help to connect the volcanic with the Plutonic formations, which will be treated of in