STRUCTURAL GEOLOGY. (continued 2)

Syenite.—This is a much abused term, but, as now employed by the best lithologists, it is the name of a rock having a single essential constituent, viz., orthoclase. Syenite in its simplest variety contains nothing but orthoclase; but in addition we usually have either hornblende, forming hornblendic syenite, or mica, forming micaceous syenite.

Syenite, it will be observed, is equivalent to gneiss with the quartz removed; but, while gneiss is the most abundant of all rocks, syenite is a comparatively rare rock; and this is simply another way of saying that nearly all orthoclase is associated with quartz. By admixture of quartz we get a perfectly gradual passage from syenite to gneiss. The orthoclase in syenite is more frequently replaced by plagioclase than it is in gneiss. In syenite, too, hornblende is much more abundant than mica; although just the opposite is true in gneiss. And, again, in gneiss the mica is principally muscovite; but in syenite it is almost exclusively biotite. Augite is a common accessory in the more basic syenite; but garnet, tourmaline, and the other accessory minerals, occurring so frequently in gneiss, are almost unknown in syenite. The specific gravity of syenite varies from 2.7 to 2.9.

Diorite.—This is a more important rock than syenite; but it is of analogous, though more basic, composition, containing a single essential constituent, viz., plagioclase. Any of the triclinic feldspars may occur in this rock, but oligoclase is most common. Like syenite, diorite usually contains hornblende, often in large proportion, forming hornblendic diorite, which sometimes passes into rocks composed entirely of hornblende. It also, but less frequently, contains mica, forming micaceous diorite. The mica is usually biotite, rarely muscovite. Mica and hornblende also often occur together in diorite, and the same is true of syenite and gneiss. Quartz is of common occurrence in the more acidic varieties of diorite, and augite in the more basic.

This is a good example of a basic rock, for all its normal constituents are basic; but the percentage of silica varies from 45 in those varieties richest in labradorite and augite to 60 or more in those containing more or less quartz and orthoclase. There is a corresponding change of color from dark to light, and of specific gravity from 2.7 to 3.1.

Diorite is not rich in accessory minerals; besides those already mentioned, the most important are chlorite, epidote, pyrite, and magnetite.

Few rocks are more clearly stratified than diorite, whether we consider the hand-specimen, or its relations to other formations. It is an abundant rock in New England.

Norite.—Like diorite, this is essentially a plagioclase rock; but there are, nevertheless, important differences. The plagioclase in diorite is mainly the more acidic species, like oligoclase; while in norite the more basic species, such as labradorite and anorthite, predominate. Hornblende, which we have observed to be an important and rather constant constituent of diorite and syenite, is much less abundant in norite; but its place is taken by augite and the allied minerals, hypersthene and enstatite. Black mica is common in norite; but white mica, orthoclase, and quartz rarely occur.

Norite is the most basic of all the feldspathic rocks, as gneiss is the most acidic; while syenite and diorite stand as connecting links, forming a gradual passage between the two extremes. Thus, in passing from gneiss to norite, we have observed a gradual diminution of the quartz, a gradual change in feldspar from orthoclase to the most basic plagioclase; at first a gradual increase in hornblende, and then a gradual change from hornblende to augite; and, finally, a gradual substitution of black mica for white. The amount of silica has decreased over 40 per cent.; and the specific gravity has increased from 2.5 in the lightest gneiss to at least 3.2 in the heaviest norite. We have also passed from light colored rocks to dark; and from those resisting atmospheric action to those easily decomposed.

The most characteristic accessory constituents of norite, besides those already mentioned, are magnetite and chrysolite; though garnet, serpentine, and pyrite often occur. In texture, this rock varies from compact to very coarsely crystalline. The specimen of labradorite (No. 23), from the norite of Labrador, affords some idea of the coarseness of the crystallization in much of this rock. It is not a common rock, except in certain regions, the best known of which in eastern North America are the coast of Labrador, various points in Canada north of the St. Lawrence, and the eastern border of the Adirondack Mountains. In hand-specimens, norite rarely appears stratified; but in the solid ledges the stratification is often as distinct as could be desired.

Many lithologists call the rocks here designated norite gabbro, and class them all in the eruptive division as essentially a coarse variety of diabase. In a similar manner, diorite and syenite are denied a place in the sedimentary series. But the stratified plagioclase rocks seem to have as strong a claim to recognition as gneiss.

We turn now to the important and interesting division of the non-feldspathic rocks or schists.

Mica Schist.—This is, next to gneiss, the most abundant rock in New England. Specimen 43 is a typical example, and from it we can readily learn what mica schist is. A glance suffices to show that it is chiefly composed of mica, but not entirely; for, on carefully examining the edges of the specimen, we cannot fail to see thin layers of hard, glassy quartz interwoven with the mica. The quartz layers are short and overlapping, and we have here a good illustration of the schistose texture; this is, in fact, a typical schist.

Mica schist usually consists, as in this instance, of mica and quartz; but it may be composed of mica alone; and sometimes kaolin or clay takes the place of the quartz, forming argillaceous mica schist. The mica in the latter is usually in very fine scales and rather inconspicuous, and the rock often passes into ordinary clay slate. Similarly, when the mica becomes deficient in the quartzose mica schist, a passage into ordinary quartzite is the result. A little feldspar is sometimes present in the rock, which thus passes into micaceous gneiss. Specimen 43 contains several crystals of red garnet, giving the variety garnetiferous mica schist. There is no other rock that contains such a large variety of beautiful accessory minerals as mica schist; and for the mineralogist it is one of the most attractive rocks. Few rocks are more distinctly stratified; and the stratification can usually be observed in hand-specimens. The mica in these rocks may be either muscovite or biotite, or both; but the former is most common. No rock shows a greater variation in the percentage of silica which it contains than mica schist, as we pass from varieties which are nearly all quartz to those which are nearly all mica.

Closely related to mica schist is the rock now known as hydromica schist, in which the ordinary anhydrous micas are replaced by hydromica. It is distinguished from mica schist by being somewhat softer, less harsh to the touch, and less lustrous. It is to be regarded usually as an incipient mica schist, which has not yet become anhydrous; though it may sometimes be just the reverse; viz.: an old mica schist which has become hydrated through the action of meteoric waters. It contains fewer accessory minerals than mica schist.

Hornblende Schist.—This is a stratified aggregate of hornblende and quartz. The quartz is granular and in thin layers, as in mica schist; but the micaceous structure is wanting, and consequently the rock does not cleave readily in the direction of the bedding. The hornblende is mostly finely crystalline, but sometimes occurs in large, bladed crystals. Garnet and some other minerals are of common occurrence in the rock; but it is not rich in accessories like mica schist. The chief difficulty in recognizing this rock consists in determining whether the white mineral is all quartz or partly feldspar. In the latter case, of course, it becomes a hornblendic gneiss.

Amphibolite (Hornblende Rock).—This is the name applied to a rock having hornblende as its sole essential constituent. Hornblende schist sometimes passes into amphibolite, through the absence of quartz; and so does diorite, when the feldspar is deficient or wanting. Specimen 20, though small, is a typical example of this rock. The physical and chemical characteristics are essentially the same as for the mineral hornblende. The texture varies from coarsely to finely crystalline. The crystals are usually short and thick, and lie in all directions in the rock, which is thus very massive, the schistose texture being entirely wanting, and the stratification rarely showing in small masses. Biotite is a common accessory in amphibolite, and garnet and magnetite frequently occur.

By the substitution of augite for hornblende, in the description of amphibolite, we get the much rarer, but otherwise very similar, rock, pyroxenite.

Talc Schist (Steatite or Soapstone).—Although not abundant, this is a useful and familiar rock. The composition is implied in the name; and by comparing it with the specimen of talc (No. 58) we can readily see that they are essentially identical. Typical talc schist is pure talc; but the talc is often mixed with more or less quartz or feldspar; and mica, chlorite, hornblende, garnet, and other minerals are of common occurrence.

This rock embraces two distinct varieties, the massive and the schistose, or foliated. The former is the common soapstone (specimen 71), which is a confused mass of crystals lying in all directions, and with no visible stratification in the small mass. In the latter, as in specimen , the talc scales lie in parallel planes, giving the rock a micaceous structure, and causing it to split easily in the direction of the stratification. The cleavage surfaces are often wavy or corrugated; and the same is true of all schistose rocks. Talc schist is easily distinguished from all other rocks by its light-grayish or greenish color, combined with its extreme softness, and its smooth, slippery feel.

Chlorite Schist.—The one essential constituent of this rock is chlorite, and the mineral specimen (No. 26) answers equally well as an example of the rock. As with talc schist, quartz, feldspar, and hydromica are rarely entirely absent. Besides these, the principal accessories are hornblende, magnetite, garnet, and epidote. This rock also agrees with talc schist in presenting two principal varieties, the massive and the schistose. It is easily distinguished from talc schist by its darker color and streak, which are very characteristic; while its green color, softness, and unctuous feel separate it from all other rocks.

This is the most basic of all the silicate rocks; but, in consequence of containing a large proportion of water, it is not the heaviest. It is, in fact, interesting and important to observe that all these hydrous silicate rocks—talc schist, chlorite schist, greensand, and serpentine—are distinctly lighter in each case than anhydrous rocks containing the same proportion of silica. They are also notable, as a class, for their softness, smooth feel, and green color.

Serpentine.—As the name implies, this rock is simply the mineral serpentine occurring in large masses, and its characteristics are precisely the same. It is fine-grained, massive, compact, rather soft, but very tough, and varies in color from very dark green to light greenish-yellow. The dark colors predominate, and specimen 25 is a typical example.

Serpentine is often intimately associated with limestone and dolomite. The white veins running irregularly through the variety known as Verd Antique Marble, however, are not calcite, as commonly supposed, but magnesite. They do not effervesce freely with cold, dilute acid, for the entire rock is magnesian, and it is probable have been at one time simply cracks along which water holding carbon dioxide has penetrated, changing the magnesia from a silicate to a carbonate.

Geologists were, at one time, almost unanimous in the opinion that all serpentine is of eruptive origin; but now it is conceded by the great majority to be in some cases a sedimentary rock. It is found interstratified with gneiss, limestone, all the schists, and many other stratified rocks. When occupying the position of an eruptive it is never an original rock; but has been formed by the alteration, in situ, of some basic anhydrous rock, most commonly olivine basalt.

Greensand.—This rock (specimen 27) consists chiefly of the mineral glauconite, mingled usually with more or less sand, clay, or calcareous matter. It is usually very friable, or in an entirely unconsolidated state. It is most abundant in the newer geological formations, especially the Cretaceous and Tertiary; and is, perhaps, the only one of the stratified silicate rocks now forming on an extensive scale in the ocean. Its value as a fertilizer, for which purpose it is extensively employed, is due to the potash that it contains.

Following is a systematic summary of the mineralogical composition of the rocks of this great division of silicates; and this, combined with the classification on page 69, presents in a condensed form all the more important facts contained in the preceding descriptions. Only the more constant and normal constituents of the species are enumerated in each case:—

Names of Species.Constituent Minerals.

⎧ Orthoclase and Quartz.

Gneiss ⎨ Orthoclase, Quartz, and Mica.

⎩ Orthoclase, Quartz, and Hornblende.

⎧ Orthoclase.

Syenite ⎨ Orthoclase and Hornblende.

⎩ Orthoclase and Mica.

⎧ Plagioclase (chiefly Oligoclase).

Diorite ⎨ Plagioclase and Hornblende.

⎩ Plagioclase and Mica.

⎧ Plagioclase (chiefly Labradorite).

Norite ⎨ Plagioclase and Augite (Diallage).

⎩ Plagioclase and Mica.

⎧ Mica.

Mica Schist ⎨ Mica and Quartz.

⎩ Mica and Kaolin.

Hornblende Schist Hornblende and Quartz.

Amphibolite Hornblende.

Pyroxenite Pyroxene.

Talc Schist Talc.

Chlorite Schist Chlorite.

Serpentine Serpentine.

Greensand Glauconite.

2. Eruptive or Unstratified Rocks.

The rocks of this great class are formed by the cooling and solidification of materials that have come up from a great depth in the earth’s crust in a melted and highly heated condition. When the fissures in the earth’s crust reach down to the great reservoirs of liquid rock, and the latter wells up and overflows on the surface, forming a volcano, then we may, as was pointed out on page 33, divide the eruptive mass into two parts: first, that which has actually flowed out on the surface, and cooled and solidified in contact with the air, forming a lava flow; second, that which has failed to reach the surface, but cooled and solidified in the fissure, forming a dike.

Lava flows or volcanic rocks and dikes or plutonic rocks are identical in composition; but there is a vast difference in texture, due to the widely different conditions under which the rocks have solidified. The dike or fissure rocks solidify under enormous pressure, and this makes them heavy and solid—free from pores. They are surrounded on all sides by warm rocks: this causes them to cool very slowly, and allows the various minerals time to crystallize. Other things being equal, the slower the cooling the coarser the crystallization; and hence, the greater the depth below the surface at which the cooling takes place, the coarser the crystallization.

The volcanic rock, on the other hand, cools under very slight pressure; and the steam, which exists abundantly in nearly all igneous rocks at the time of their eruption, is able to expand, forming innumerable small vesicles or bubbles in the liquid lava; and these remain when it has become solid. Cooling in contact with the air, the lava cools quickly, and has but little chance for crystallization. Hence, to sum up the matter, we say: plutonic rocks are solid and crystalline; and volcanic rocks are usually porous or vesicular, and uncrystalline.

As we descend into the earth’s crust, it is perfectly manifest that the volcanic must shade off insensibly into the dike rocks, and we find it impossible to draw any but an arbitrary plane of division between them; but this is no argument against this classification, for, as already stated, all is gradation in geology, and we experience just the same difficulty in drawing a line between conglomerate and sandstone, or between gneiss and mica schist, as between the dike rocks and volcanic rocks.

We will now observe to what extent the distinctions between these two great classes of eruptives can be traced in the rocks themselves, beginning with the dike rocks. But first it is important to notice the general fact, clearly expressed in the classification, that, with perhaps some trifling exceptions which need not be mentioned here, all eruptive rocks are silicates, and nearly all are feldspathic silicates. They are of definite mineralogical composition, and, like the chemically and organically formed stratified rocks, can be classified chemically. But, although there are eruptives corresponding closely in composition to the feldspathic silicates, which we have just studied, we find among them little to represent the non-feldspathic silicates, and nothing corresponding in composition to the limestones, dolomites, gypsum, flint, tripolite, siliceous tufa, iron-ores, bitumens or coals.

1. Plutonic (Dike) Rocks.—These are also known as the ancient eruptive rocks, and for this reason: It is impossible, of course, for us to observe them except where they occur on or near the earth’s surface. But, since they are formed wholly below the surface, and usually at great depths in the earth, it is evident that they can appear on the surface only as the result of enormous erosion; and erosion is a slow process, demanding, in these cases, many thousands or millions of years. Therefore, the more ancient dike rocks alone are within our reach; those of recent formation being still deeply buried in the earth’s crust. It follows, as a corollary to this explanation, that the coarseness of the crystallization of any dike rock must be a rough measure of its age and of the amount of erosion which the region has suffered since its eruption.

As regards composition, the dike rocks present, as already stated, essentially the same combinations of minerals as the feldspathic silicates of the stratified series, but occurring under different physical conditions and having a widely different origin. The only important difference in texture between the two classes of rocks is that the sedimentary rocks are stratified and the dike rocks are not; and when we consider that the dike rocks sometimes present a laminated structure that resembles stratification, while the sedimentary rocks frequently appear unstratified, it is easy to understand why, in the absence of any marked difference in composition, geologists have often found it difficult to distinguish the two classes of rocks. We also find here the explanation and the justification of the fact that the names of the dike rocks are in most cases the same as those of the sedimentary rocks of similar composition.

Granite.—Granite (from the Latin granum, a grain) is a crystalline-granular rock, agreeing in composition with gneiss. The essential constituents are quartz and orthoclase; and when they alone are present we have the variety binary granite. Mica, however (commonly muscovite, sometimes biotite, and frequently both) is usually added to these, forming micaceous granite (specimen 44); and often hornblende, forming hornblendic granite (specimen 45). The orthoclase is sometimes replaced in part by triclinic species, especially albite and oligoclase. Accessory minerals are not so abundant in granite as in gneiss; but, besides those named, garnet, tourmaline, pyrite, apatite, and chlorite are most common. Orthoclase is always the predominant ingredient; and, except when there is much hornblende present, usually determines the color of the granite. Thus, specimens 44 and 45 are gray because they contain gray orthoclase; while all red granites contain red or pink orthoclase. The quartz has usually been the last of the constituents to crystallize or solidify; and, having been thus obliged to adapt itself to the contours of the orthoclase and mica, it is rarely observed in distinct crystals.

In texture, the granites vary from perfectly compact varieties, approaching petrosilex, to those which are so coarsely crystalline that single crystals of orthoclase measure several inches in length. Of course one of the most important things to be observed about granite, especially in comparing it with gneiss, is the complete absence of anything like stratification; that, as before stated, being the only important distinction between the two rocks. Gneiss is the most abundant of all stratified rocks, and granite stands in the same relation to the eruptive series.

Syenite.—This is an instance where stratified and eruptive rocks, agreeing in composition, have the same name. That rocks consisting of orthoclase, of orthoclase and hornblende, or of orthoclase and mica, i.e., having the composition of syenite, do occur in both the eruptive and stratified series there can be no doubt. They should, however, have distinct names on account of their unlike origins; and would have but for the practical difficulty in determining, in many cases, whether the rock is stratified or not. The best that we can do now, when we desire to be specific, and have the necessary information, is to say stratified syenite or eruptive syenite, as the case may be.

Diorite.—Here, again, we find identity of names, as well as of composition, between the two great series. Eruptive diorite is an abundant and well known rock, and consists of the same minerals as stratified diorite combined in the same proportions. Diorite includes a large part of the dike rocks commonly known as “trap” and “greenstone.” The principal accessories are chlorite, epidote, pyrite, magnetite, apatite, and quartz. The texture varies from perfectly compact or felsitic to coarsely crystalline; averaging, however, less coarse than syenite and granite.

Diabase.—By referring to the classification it will be seen that diabase occupies the same position among the dike rocks as norite among the stratified rocks. Like norite it consists usually of the more basic varieties of plagioclase with or without augite, diallage, or hypersthene. Augite, or one of its representatives, is usually present, and is often the principal constituent. Specimen 1 shows a somewhat equal development of the feldspar and augite. The name gabbro is sometimes applied to the coarser and more feldspathic diabases, and especially to those containing diallage or hypersthene in the place of common augite. In the opinion of some high authorities, however, it is unnecessary to recognize two species here; and it makes the classification more simple and symmetrical not to do it. The principal accessories in diabase are biotite, chlorite, magnetite, pyrite, calcite, and olivine. Chlorite is often an important constituent, giving the rock a greenish aspect; but here, as well as in diorite, the chlorite is due chiefly or entirely to the alteration of the augite and feldspar; and the chloritic varieties of diorite and diabase together make up the old species “greenstone.” Similarly, the more compact and darker varieties of these two rocks, forming regular, wall-like dikes, are known as “trap.” Specimen 46.

In consequence of their more basic composition, diabase and diorite are usually strongly contrasted with granite and syenite in color and specific gravity, being darker and heavier. The basic rocks, too, decay much more readily than the acidic.

2. Volcanic Rocks.—As regards composition, we shall find nothing new in the volcanic series; for the rocks of this group present essentially the same combination of minerals as the dike rocks. In composition, the dike and volcanic rocks are identical; but in texture, as already explained, there is a vast difference. The volcanic rocks differ so widely in texture from both the dike and stratified species, that there is rarely any difficulty in distinguishing them; and hence they have in every instance distinct names.

Volcanic rocks are rarely found in this part of the world; and specimens of most of them are difficult to obtain. For this reason they can only be noticed briefly here, since it is the plan of this Guide to give especial attention only to those portions of the subject which can be illustrated by material within easy reach of teachers.

Rhyolite.—This rock corresponds in composition with granite and gneiss, but is less frequently micaceous. The orthoclase in rhyolite, and generally in volcanic rocks, is the clear, pellucid variety—sanidine. It is more difficult to separate from quartz than ordinary orthoclase, the chief distinguishing feature being its cleavage. Plagioclase and hornblende are common, but not abundant, constituents. The mica, when present, is usually biotite. The texture of rhyolite is often more or less distinctly porphyritic, having a finely crystalline or granular matrix, with interspersed crystals of sanidine and quartz. The rock has usually a rough, harsh feel; and while the coarser varieties have the aspect of granite, the finer approach petrosilex; but all are somewhat porous, which is seen in the lower specific gravity of rhyolite as compared with granite and gneiss.

Trachyte.—In texture and general aspect rhyolite and trachyte are nearly identical. Trachyte, however, is darker, contains little or no quartz, and more hornblende and plagioclase. In fact, it agrees in composition with syenite. This is one of the most important of the volcanic rocks.

Obsidian.—Obsidian is sharply distinguished from all other rocks by its perfect vitreous texture; it is a true volcanic glass. Its surface (specimen 47) is smooth and glassy, and its fracture eminently conchoidal. To the naked eye, and usually under the microscope, the typical variety is perfectly homogeneous; chemical analysis, however, shows that it has the composition, commonly of rhyolite, but sometimes of trachyte. Obsidian is, in fact, simply rhyolite or trachyte which, cooling quickly, has not had time to crystallize, but has remained permanently in the amorphous or glassy state. The composition is sometimes partially revealed where a portion of the sanidine comes out in distinct crystals porphyritically interspersed through the glass. The homogeneity of the texture is sometimes disturbed: by numerous minute concentric cracks, forming what is known as perlitic structure and the variety perlite; by numerous small spherical concretions, forming the spherulitic structure and the variety spherulite; and also by the banding, which is the result of flowing while in a plastic state, whereby portions of the glass of slightly different colors are drawn out into layers and interlaminated. The bands are rarely continuous for any distance, being usually merely elongated lenticular streaks. The glassy state is generally one of inferior density, and hence we find that obsidian is lighter than the crystalline rocks of the same composition. Obsidian is a good illustration of a non-essential color, for its capacity and jet-black color are due entirely to impurities. In very thin flakes it is transparent and white. It also forms a white powder when crushed, i.e., it has a white streak.

Obsidian is often vesicular, from the expansion of the steam and other gases which it contained when liquid. The most thoroughly vesicular varieties are known as pumice (specimen 48). The vesicular texture, by rendering the rock impervious to light, conceals the impurities, and thus we get a snow-white pumice from black obsidian. The vesicles are frequently elongated, sometimes in a definite direction, though often forming an irregular net-work of glassy fibres. Pumice is often light enough to float on water, and it is transported thousands of miles by the oceanic currents. It is employed in the arts, and good specimens can be obtained at almost any drug-store.

Petrosilex and Felsite.—Sharply defined groups are unknown in lithology, but all is gradation; and between rhyolite and trachyte, which are always more or less distinctly crystalline, and obsidian, which is a true glass and perfectly amorphous, there is no break. It is impossible to draw a sharp line and say, Here the vitreous texture ends and the crystalline begins; for the transition is not abrupt, but gradual. We recognize, really, in these feldspathic rocks, an intermediate state, which is neither crystalline nor colloid, but both; and this lithologists have designated the felsitic texture. Felsitic matter cannot, even with the highest powers of the microscope, be resolved into separate grains or particles; and it does not exhibit, except perhaps very indistinctly, the phenomenon of double refraction. In other words, it is not truly crystalline or stony, and yet it is just as clearly not amorphous or glassy.

Feldspathic rocks exhibiting the felsitic texture in whole or in part are known as felsites. Many high authorities hold that true felsites are found only among the eruptive rocks; while others claim that they are in part, or wholly, of sedimentary origin. The writer accepts the former view. The felsites are in part acid lavas which have cooled too slowly to form a true glass, like obsidian, and yet too quickly to become truly crystalline, like rhyolite and trachyte. But they are also in large part simply devitrified obsidian. Glass is an unstable form of mineral matter; and every species of glass, including obsidian, tends with the lapse of time to become crystalline or stony, the amorphous changing to the felsitic structure. Thus, in many cases or usually, what we now call felsites were originally true glassy obsidian. Being perfectly intimate mixtures of the component minerals, the composition of felsites can usually be determined with certainty only by means of chemical analysis. By this means chiefly, it has been proved that there are felsites agreeing in composition with both rhyolite and trachyte. There is this general difference in composition, however, between these crystalline rocks and the felsites; viz.: mica, hornblende, and augite are generally wanting in the latter. From this it follows that the felsites are, with unimportant exceptions, composed either of quartz and feldspar or of feldspar alone.

The physical differences between the felsites of unlike composition are not great; but they are sufficient to warrant the division of the felsites into two species: a basic species, to which the term felsites may properly be restricted; and an acidic species, for which petrosilex is a very appropriate name. According to this arrangement, felsite is composed chiefly of orthoclase, and, as the table shows, agrees in composition with trachyte; while petrosilex consists mainly of orthoclase and quartz, agreeing in composition with rhyolite. We find here nothing new in composition; but petrosilex and felsite are simply the crystalline rocks which we have already studied, repeated under a different texture.

The typical felsite or petrosilex is composed entirely of felsitic matter, and is perfectly homogeneous, like flint or jasper, which it closely resembles in hardness and other physical characteristics. As a rule, however, the rock is not entirely homogeneous, but there is a manifest tendency in the component minerals, and especially in the feldspar, to separate out, usually in the form of crystals. In the banded variety (specimen 42) the rock is built up of thin layers, which are often alternately quartzose and feldspathic. There is not a perfect separation of the minerals; but that the quartz is chiefly in the dark layers, and the feldspar in the light, is shown by the way in which the layers are affected by the weather.

One of the most common varieties is where a portion, frequently a large portion, of the feldspar comes out in the form of distinct, separate crystals, producing a porphyritic texture. Specimens 5, 6, and 7 are examples of porphyritic felsite; and after examining these we can no longer doubt that feldspar is an important constituent of the rock. Petrosilex and felsite are more generally porphyritic than any other rocks; and they are commonly called porphyry. It is better, however, since almost any rock may be porphyritic, and since this texture cannot be correlated with any particular composition, not to use porphyry as a rock-name, but simply as the name of a very important rock-texture. The banded and porphyritic textures are about equally characteristic of petrosilex and felsite. In petrosilex, quartz, as well as feldspar, is sometimes porphyritically developed, forming the variety known as quartz-porphyry. There is no limit to the proportion of the quartz and feldspar which may crystallize out in this way, and thus we find a perfectly gradual passage from normal petrosilex or felsite to thoroughly crystalline granite and syenite.

Andesite.—This rock has nearly the texture of rhyolite and trachyte, but is darker and heavier, and corresponds in composition to diorite, consisting of plagioclase and hornblende, with usually more or less sanidine, quartz, augite, biotite, and magnetite.

Basalt.—The rock bearing this familiar name represents diabase among the dike rocks. It is the most basic of the volcanic rocks, and consists of the more basic varieties of plagioclase, especially labradorite, with augite, magnetite, and titanic iron. Olivine is a very common and characteristic constituent, and the plagioclase is often replaced in part by leucite and nephelite. The basalts are usually black, and of high specific gravity; and vary in texture from compact to coarsely crystalline. The contraction due to cooling frequently results in the development of a columnar structure of remarkable regularity, the columns being normally hexagonal and standing perpendicularly to the cooling surfaces of the mass. This structure occurs in other eruptive rocks, but is most characteristic of basalt.

Tachylite.—Tachylite is a highly basic volcanic glass, standing in the same relation to basalt and andesite that obsidian does to trachyte and rhyolite. It is much heavier than obsidian, and is perfectly black and opaque, except in the finest fibres. It is a comparatively rare rock, for the reason that basalt and andesite crystallize more readily than the acidic rocks on passing from the liquid to the solid state. On the surface of the basic lava, however, where it is in contact with the air, and congeals almost instantly, a film of glass is formed; but this may not be more than a small fraction of an inch in thickness. Like obsidian, tachylite is often vesicular; but the vesicular basic rocks, as well as the solid, are usually stony. They occur in vast abundance in many volcanic regions, and may be considered the typical lava (specimen 49).

In the more ancient lavas, the vesicles are frequently filled by various minerals—chlorite, epidote, quartz, calcite, etc.—deposited by infiltrating waters, and derived in most cases from the decomposition of the original constituents of the rock. Thus the vesicular is changed to the amygdaloidal texture, and the lava becomes an amygdaloid (specimen 50). The amygdaloidal texture is common in the basic lavas and rare in pumice, simply because the former are more readily decomposed and contain a greater variety of bases from which secondary minerals can be formed.

Porphyrite and Melaphyr.—These two rocks hold essentially the same relation as regards origin and structure to the basic lavas that petrosilex and felsite do to the acidic lavas. Porphyrite agrees in composition with andesite, and melaphyr with basalt. They are usually dark-colored rocks having a compact or felsitic texture. Porphyrite is, as the name implies, very commonly porphyritic; while melaphyr is often vesicular or brecciated, exhibiting all the structural features of tachylite and basalt, and being in its older forms very generally amygdaloidal.

Volcanic Tuff and Agglomerate.—Besides the crystalline, glassy, and felsitic lavas, already described, and due chiefly to the rate of cooling of the liquid rock, we may recognize a fourth class to include the very abundant lavas which, during explosive eruptions, are ejected in the solid state, being violently blown out of the crater in the form of dust and fragments. Falling on the slopes of the volcano or over the surrounding country, as in the case of the buried city of Pompeii, the fragmental lavas remain largely unstratified. But when, as frequently happens, they fall into the sea, they are assorted by the waves and currents and arranged in layers after the manner of ordinary sediments, with which they are often more or less mixed. Before they become consolidated the finer fragmental lava, of whatever composition, is called volcanic dust, and the coarser lapilli or volcanic sand; while the consolidated materials are known as tuff and agglomerate respectively.