Part 9

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.

SUPPLEMENT TO LITHOLOGY.

VEIN ROCKS.

All rocks are not embraced in the sedimentary and eruptive divisions, but there is a third grand division, which, although rarely mentioned or recognized in the more comprehensive works on geology, it is deemed best not to leave entirely unnoticed here. These are the _vein_ rocks. They present an immense number of varieties, and yet, taken altogether, form but a small fraction of the earth’s crust. They are, however, the great repositories of the precious and other metals, and hence are objects of far greater interest to the miner and practical man than the eruptive rocks, or, in some parts of the world, even than the sedimentary rocks.

The vein rocks, like the eruptive rocks, occupy fissures in the earth’s crust intersecting the stratified formations; but the fissures filled with vein rocks are called veins, and not dikes. We will first notice the mode of formation of a typical vein, and then examine its contents. Geologists are agreed that water penetrates to a very great depth in the earth’s crust. All minerals are more or less soluble in water; and we may consider the water circulating through the rocks, especially at considerable depths, as, in most cases, a saturated solution of the various minerals of which they are composed. Very slight changes in the conditions will cause saturated solutions to deposit part of their mineral load. The water at great depths has a high temperature, and is subjected to an enormous pressure; and both of these circumstances favor solution. Suppose, now, that these hot subterranean waters enter a fissure in the crust and flow upwards, perhaps issuing on the surface as a warm mineral spring; as they approach the surface, the temperature and pressure, and consequently their solvent power, are diminished; and a portion of the dissolved minerals must be deposited on the walls of the fissure, which thus becomes narrower, and in the course of time is gradually filled up. The vein is then complete; and the mineral waters are forced to seek a new outlet.

Veins have the same general forms as dikes, since the fissures are formed in the same way for both; but the vein is of slow growth, and may require ages for its completion, while the dike is formed in an hour or a day. It is now generally believed that water is an important agent in the formation of eruptive rocks; since they all contain water at the time of their eruption; and since it has been demonstrated that, while ordinary rocks require a temperature of 2000° to 3000° for their fusion in the absence of water, they are liquified at temperatures below 1000° in the presence of water. In other words, common rocks are very infusible and insoluble bodies, and heat and water acting independently have little effect upon them; but when fire and water are combined in what is now known as aqueo-igneous fusion, they prove very efficient agents of liquefaction.

If we adopt these views, then it can be shown that, in origin, veins and dikes differ in degree only, and are not fundamentally unlike; and the formation and relations of the three great classes of rocks may be summarized as follows:—

The ocean and atmosphere, operating on the earth’s surface, have worked over and stratified the crust, until the sedimentary rocks have now an average thickness variously estimated at from ten to thirty or forty miles. This entire thickness of stratified rocks, and a considerable depth of the underlying unstratified crust, must be saturated with water; and all but the more superficial portions of this water-soaked crust must be very hot, the temperature increasing steadily downwards from the surface. Both eruptive and vein rocks originate in this highly heated, hydrated crust. Eruptive rocks are formed when the heat, aided by more or less water, softens the rocks, either stratified or unstratified, by aqueo-igneous fusion, and the plastic materials are forced up through fissures to or toward the surface. Vein rocks are formed when the water, aided by more or less heat, dissolves the rocks, either stratified or unstratified, by what may be called igneo-aqueous solution, and subsequently deposits the mineral matter in, _i.e._, on the walls of, fissures leading up to or toward the surface. In the case of the dike rocks, heat is the chief agent, and water merely an auxiliary; while with the vein rocks it is just the reverse. But between the two it is probably impossible to draw any sharp line.

The water circulating through the crust, and saturated with its various mineral constituents, has been called the “juice” of the crust; and veins are formed by the concentration of this earth-juice in fissures. One of the most important characteristics of the vein rocks, as a class, is the immense variety which they present; for nearly every known mineral is embraced among their constituents; and these are combined in all possible ways and proportions, so that the number of combinations is almost endless. The solvent power of the subterranean waters varies for different minerals; and appears often to be greatest for the rarer species. In other words, there is a sort of selective action, whereby many minerals which exist in stratified and eruptive rocks, so thinly diffused as to entirely escape the most refined observation, are concentrated in veins in masses of sensible size; and our lists of known minerals and chemical elements are undoubtedly much longer than they would be if these wonderful storehouses of fine minerals which we call veins had never been explored. As a rule, the minerals in veins form larger and more perfect crystals than we find in either of the other great classes of rocks. This is simply because the conditions are more favorable for crystallization in veins than in dikes or sedimentary strata. In both dike and stratified rocks, the growing crystals are surrounded on all sides by solid or semi-solid matter; and, being thus hampered, it is simply impossible that they should become either large or perfect. In the vein, on the other hand, there are usually no such obstacles to be overcome; but the crystals, starting from the walls of the fissure, grow toward its centre, their growing ends projecting into a free space, where they have freedom to develop their normal forms and to attain a size limited only, in many cases, by the breadth of the fissure. With, possibly, some rare exceptions, all the large and perfect crystals of quartz, feldspar, mica, beryl, apatite, fluorite, and of minerals generally, which we see in mineralogical cabinets, have originated in veins. Those fissures which become the seats of mineral veins are really Nature’s laboratories for the production of rare and beautiful mineral specimens; and hence the vein rocks are the chief resort of the mineralogist, to whom they are of far greater interest than all the eruptive and stratified rocks combined.

The leading characteristics, then, of the vein rocks may be summarized as follows: (1) They contain nearly all known minerals, including many rare species and elements which are unknown outside of this class of rocks. (2) These mineral constituents, occurring singly and combined, give rise to a number of varieties of rocks so vast as to baffle detailed description. (3) They exceed all other rocks in the coarseness of their crystallization, and in the perfection and beauty of the single crystals which they afford.

PETROLOGY.

In lithology we investigate the nature of the materials composing the earth’s crust—the various minerals and aggregates of minerals, or rocks; while in petrology we consider the forms and modes of arrangement of the rock-masses,—in other words, the architecture of the earth.

Petrology is the complement of lithology, and in many respects it is the most fascinating division of geology, since in no other direction in this science are we brought constantly into such intimate relations with the beautiful and sublime in nature. The structures of rocks are the basis of nearly all natural scenery; for what we call scenery is usually merely the external expression, as developed by the powerful but delicate sculpture of the agents of erosion—rain and frost, rivers and glaciers, etc.—of the geological structure of the country. And to the practised eye of the geologist, a fine landscape is not simply a pleasantly or grandly diversified _surface_, but it has _depth_; for he reads in the superficial lineaments the structure of the rocks out of which they are carved.

But, while the magnitude of the phenomena adds greatly to the charm of the study, it also increases the difficulties and taxes the ingenuity of the teacher whose work must be done indoors. According to our ideal method, natural science ought to be taught with natural specimens; and yet here our main reliance must be upon pictures and diagrams.

Nature, however, has not been wholly unmindful of our needs; for she has worked often upon a very small as well as a very large scale; many of the grandest phenomena being repeated in miniature. Thus we observe rock-folds or arches miles in breadth and forming mountain masses, and of all sizes from that down to the minutest wrinkle. So with veins, faults, etc. And the wonderful thing is that these small examples, which may be brought into the class-room, are usually, except in size, exactly like the large. Now the aim of every teacher in this department should be to secure a collection of these natural models. It is not an easy thing to do, except one has plenty of time; for they can rarely be purchased of dealers, but must usually come as the choicest fruit of repeated excursions to the natural ledges and quarries, the seashore and the mountains. But for the difficulty of getting the specimens there is some compensation, since it may be truly said that for the _collector_ specimens obtained in this way have an interest, a value, and a power of instruction beyond what they would otherwise possess.

_Classification of Structures._

The structures of rock divide, at the outset, into two classes:—(1) the _original structures_, or those produced at the same time and by the same forces as the rocks themselves, and which are, therefore, peculiar to the class of rocks in which they occur (_e.g._ stratification, ripple-marks, fossils, etc.); and (2) the _subsequent structures_, or those developed in rocks subsequently to their formation, and by forces that act more or less uniformly upon all classes of rocks, and which are, therefore, in a large degree, common to all kinds of rocks (_e.g._ folds, faults, joints, etc.).

The original structures are conveniently and naturally classified in accordance with the three great classes of rocks: (1) stratified rocks, (2) eruptive rocks, and (3) vein rocks; while the subsequent structures, not being peculiar to particular classes of rocks, are properly divided into those produced by (1) the subterranean or igneous agencies, and (2) the superficial or aqueous agencies.

Original Structures of Stratified Rocks.

STRATIFICATION.—All rocks formed by strewing materials in water, and their deposition in successive, parallel, horizontal layers, are _stratified_; and this structure is their _stratification_. It is the most important of all rock structures; and there is no kind of structure the origin of which is more fully or certainly known. The deposition of sediment in carefully assorted horizontal layers is readily brought within the comprehension of children by simple experiments with sand and clay in water; and still better by the examination of the deposits formed in roadside pools during heavy rains (Fig. 1), and by digging into beaches and sandbars, which every child will recognize as formed of materials arranged by water. Great stress should be laid upon the fact that a lake like Erie or Champlain is simply a large pool with several more or less turbid streams flowing into it, while the single stream flowing out is clear, the sediment having evidently been deposited in the lake; and that every lake is, like the roadside pool, being gradually filled up with sedimentary or stratified rocks. But the ocean is a still larger pool, receiving mud and sand from many streams; and since we know that nothing escapes from the ocean but invisible vapor, it is plain that the mud and sand and all other kinds of sediment carried into the ocean must be deposited on its floor, and chiefly, as we have seen, on that part nearest the land. The consolidation of beaches, bars, and mud-flats is all that is necessary to convert them into stratified formations of conglomerate sandstone and slate.

Let us notice now, more particularly, the causes of visible stratification. As we can easily prove by an experiment with clay in a bottle of water, if the same kind of material is deposited continuously there will be no visible stratification in the deposit. It will be as truly stratified as any formation, but not visibly so; because there is nothing in the nature of the material or the way in which it is laid down to bring out distinct lines of stratification. Continuous and uniform deposition obtains very frequently in nature, but rarely continues long enough to permit the formation of thick beds or strata. Hence, while the stratification is almost always visible on the large surfaces of sandstone, slate, etc., exposed in quarries and railway cuttings, and may usually be seen in the quarried blocks, it is often not apparent in hand specimens, which may represent a single homogeneous layer. There is one important exception, and that is where the particles, although of the same kind, are flat or elongated. Pebbles of these forms are common on many beaches; and since they are necessarily arranged horizontally by the action of the water, they will, by their parallelism, make the stratification of the pudding-stone visible. The same result is accomplished still more distinctly by the mica scales, etc., in sandstone and slates, the leaves and flattened stems of vegetation in bituminous coal, and the flat shells in limestone.

In all other cases, visible stratification implies some change in the conditions; either the deposition was interrupted, or different kinds of material were deposited at different times. The first cause produces planes of easy splitting, or fissility, especially in fine-grained rocks, like shale. This shaly structure or lamination-cleavage may be due, in some cases, to pressure, but it is commonly understood to mean that each thin layer of clay became partially consolidated before the next one was deposited upon it, so that the two could not perfectly cohere. Parallel planes of easy splitting are, however, by themselves, of little value as indications of stratification, since the lamination-cleavage is not easily distinguished from slaty-cleavage (roofing slate) and parallel jointing, structures developed subsequently to the deposition of sediments and quite independent of the stratification. The second cause, or variations in the kind of sediment, gives alternating layers differing in color, texture, or composition, as is seen frequently in sandstone, slate, gneiss, etc.; and of all the indications of stratification these are the most important and reliable.

A layer composed throughout of essentially the same kind of rock, as conglomerate or sandstone, and showing no marked planes of division, is usually regarded as one _bed_ or _stratum_, although it may vary considerably in texture or color; while the thinner portions composing the stratum and differing slightly in color, texture, and composition, and the thin sheets into which shaly rocks split, are the _laminæ_ or _leaves_. In Fig. 2 the strata are designated by letters, and the fine lines and rows of dots show the constituent laminæ, while the whole section may be regarded as a small part of a great geological formation. The geological record is written chiefly in the sedimentary rocks; and the formations, strata, and laminæ may be regarded as the volumes, chapters, and pages in the history of the earth. Now every feature of a rock, lithological or petrological, finds its highest interest in the light which it throws upon the history of the rock, _i.e._, upon the conditions of its formation. Observe what the section in Fig. 2 teaches concerning the geological history of that locality; premising that any chapter of geological history written in the stratified rocks should be read from the bottom upwards, since the lowest strata must have been formed first and the highest last. The lowest stratum exposed is conglomerate, indicating a shingle beach swept by strong currents which carried away the finer material. Upwards, the conglomerate becomes finer and shades off into sandstone, and finally into shale, showing that the water has become gradually deeper and more tranquil, the shore having, in consequence of the subsidence, advanced toward the land. The next two strata show that this movement is probably reversed; at any rate, the currents become stronger again, and the shale passes gradually into sandstone and conglomerate. The beach condition prevails now for a long time, and thick beds of sand and gravel are formed. The sea then deepens again, and we observe a third passage from coarse to fine sediment. This locality is now remote from the shore, the gentle currents bringing only the finest mud, which slowly builds up the thick bed of shale, in the upper part of which shells are abundant, indicating that the deposition of mechanical sediment has almost ceased, and that the shale is changing to limestone. The purity of the limestone, and the crinoids and other marine organisms which it contains, prove that this has now become the deep, clear sea; and this condition is maintained for a long period, for the limestone is very thick, and this rock is formed with extreme slowness.

The most important point to be gained here is that every line of stratification and every change in the character of the sediments is due to some change of corresponding magnitude in the conditions under which the rock was formed. The slight and local changes in the conditions occur frequently and mark off the individual laminæ and strata, while the more important and wide-spread changes determine the boundaries of the groups of strata and the formations.

Strata are subject to constant lateral changes in texture and composition, _i.e._, a bed or formation rarely holds the same lithological characteristics over an extended area. There are some striking exceptions, especially among the finer-grained rocks, like slate, limestone, and coal, which have been deposited under uniform conditions over wide areas. It is the general rule, however, particularly with the coarse-grained rocks, which have been deposited in shallow water near the land, that the same continuous stratum undergoes great changes in thickness and lithological character when followed horizontally. A stratum of conglomerate becomes finer grained and gradually changes into sandstone, which shades off imperceptibly into slate, and slate into limestone, etc. Where the stratum is conglomerate, its thickness will usually be much greater and more variable than where it is composed of the finer sediments. The rapidity of these changes in certain cases is well shown by the parallel sections in Fig. 3. These represent precisely the same beds, as the connecting lines indicate, at points only twenty feet apart.

When we glance at the conditions under which stratified rocks are now being formed, it is plain that all strata must terminate at the margin of the sea in which they were deposited, and in the marginal portions of that sea, especially, must exhibit frequent and rapid changes in composition, etc. The sediments forming the surface of the sea-bottom at the present time may be regarded as belonging to one continuous stratum; and it is instructive to examine a chart of any part of our coast, such as Massachusetts Bay, on which the nature of the bottom is indicated for each sounding, and observe the distribution of the different kinds of sediment. On an irregular coast like this, especially, the gravel, sand, and mud of different colors and textures, and the different kinds of shelly bottom, form a patchwork, the patches being, for the most part, of limited extent and shading off gradually into each other.