Chapter II: Igneous Agencies (4)

On a more regular coast, like that of New Jersey, the sediments are distributed with corresponding uniformity, the changes are less frequent and more gradual, and we have here a better chance to observe the normal arrangement of the sediments along a line from the shore seawards—gravel, sand, mud, and shells. On the beach we find the shingle and coarse pebbles, shading off rapidly into fine pebbles and sand. The zone or belt of sandy bottom may vary in width from a mile or two to twenty miles or more, becoming gradually finer and changing into clay or mud, which covers, usually, a much broader zone, sometimes extending into the deeper parts of the sea, but gradually giving way to calcareous sediments. Hence we may say that the finer the sediment the greater the area over which it is spread; but, on the other hand, the coarser the sediment the more rapidly it increases in thickness. In other words, the horizontal extent of a formation deposited in any given period of time is inversely, and the vertical extent or thickness is directly, proportional to the size of the particles.

Observations made in deep wells and mines, and where, by upturning and erosion, the edges of the strata are exposed on the surface, show that the vertical order of the different kinds of sedimentary rocks in the earth’s crust is extremely variable. But when we take a general view of a great formation, it is often apparent that it consists chiefly of coarse-grained rocks in the lower part and fine-grained rocks in the upper part. This is, in general, a necessary consequence of the fact that a great thickness of sediments can only be formed on a subsiding sea-floor. Such a formation must consist chiefly of shore deposits, and be deposited near the shore where the sea is shallow. Hence, 10,000 feet of sediments implies nearly that amount of subsidence. In consequence, the shore line and the several zones of sediment advance towards the land; and sand is deposited where gravel was at first, and as the subsidence continues, both clay and limestone are finally deposited over the original beach. When the sea-floor rises, the order of the sediments is reversed; and it will be observed that in consequence of the advance and retreat of the shore-line, the formations grow edgewise to a considerable extent.

OVERLAP AND INTERPOSITION OF STRATA.—Another consequence of the constant oscillation of the shoreline is that successive deposits in the same sea will often cover different and unequal areas. When, in consequence of subsidence, one formation extends beyond and covers the edge of another, as shown in Fig. 4, we have the phenomenon described as overlap. Interposition is similar, being the case where a formation (Fig. 5, _c._) does not, in certain directions, cover so wide an area as the strata (_b. d._) above and below it, which are thus sometimes found in contact, although normally separated by the entire thickness of the intermediate and, seemingly, interposed stratum.

UNCONFORMABILITY.—We have already seen that the rocks on the land are being constantly worn away by the agents of erosion; and it is also a matter of common observation that the strata thus exposed are often not horizontal, but highly inclined, having been greatly disturbed and crumpled during their elevation. Now, when such a land-surface subsides to form the sea-bottom, and new strata are spread horizontally over it, they will lie across the upturned and eroded edges of the older rocks, and fill the hollows worn out of the latter, as shown in Fig. 6; and the new formation is then said to rest unconformably upon the older. Two strata or formations are unconformable when the older has suffered erosion (Fig. 6), or both disturbance and erosion (Fig. 4) before the deposition of the newer.

When strata are conformable, the deposition may be presumed to have been nearly or quite continuous; but unconformability clearly proves a prolonged interruption of the deposition during which the elevation, erosion, and subsidence of the sea-bottom took place. The section in Fig. 7 shows a second unconformability, proving that the sea-bottom has here been lifted three times to form dry land. An unconformability may sometimes be clearly established when the actual contact of the two formations cannot be seen, as where the new formation is a conglomerate containing fragments of the older.

IRREGULARITIES OF STRATIFICATION.—These are especially noticeable in sandstone and conglomerate, which have been deposited chiefly by strong, local, and variable currents; the kind and quantity of sediment, of course, varying with the strength and direction of the current. Two kinds of irregularity only may be specially noticed here: (1) contemporaneous erosion and deposit, where, in consequence of a change in the currents, fine material recently deposited is partially swept away and its place taken by coarser sediments; and (2) oblique lamination, or current-bedding, where the strata are horizontal as usual, but the component laminæ are inclined at various angles. This structure is characteristic of sediments swept along by strong currents, especially when deposited in shallow basins or depressions.

RIPPLE-MARKS.—All who have been on a beach or sand-bar must have noticed the lines of wavy ridges and hollows, or ripples, on the surface of the sand. These are sand-waves, produced by water moving over the sand, or by air moving over dry sand, as ordinary waves are formed by air moving over water. Each tide usually effaces the ripple-marks made by its predecessor and leaves a new series, to be obliterated by the next tide. But where sediment is constantly accumulating, a rippled surface may be gently overspread by a new layer, and thus preserved. Other series of ripples may, in like manner, be formed and preserved in overlying layers; and when the beach becomes a firm sandstone, a section of it will show the rippled surfaces almost as distinctly as when they were first formed (Fig. 8). Ripple-marks are most perfect in fine sand. They are not formed in gravel, because it is too coarse; nor in clay, because it is too tenacious. They are usually limited to shallow water; and are always regarded as proving that the rocks in which they occur are shallow-water or beach deposits. They are normally at right angles to the current that produces them, and where this changes with the direction of the wind, cross-ripples and other irregularities are introduced. Ripple-marks are also usually parallel with the beach, and when they are found in the rocks they give us the direction, as well as the position, of the ancient shore-line.

Again, the friction of the water pushes the sand-grains along, rolling them up on one side of the ripple and letting them fall down on the other. Hence ripples, formed by a current are always moving and are unsymmetrical on the cross-section, presenting a long, gentle slope toward the current, and a short, steep slope away from it, the arrow in the figure indicating the direction of the current, or of the sea in the case of a beach. And we may thus learn from the fossil ripples, in some cases, not only the position and direction of the ancient shore, but also on which side the land lay, and on which side the sea. When the water is in a state of oscillation, without any distinct current, more symmetrical ripples are produced.

RILL-MARKS, RAIN-PRINTS, AND SUN-CRACKS.—“One of the most fascinating parts of the work of a field-geologist consists in tracing the shores of former seas and lakes, and thus reconstructing the geography of successive geological periods.” His conclusions, as we have already seen, are based largely upon the nature of the sediments; but still more convincing is the evidence afforded by those superficial features of the strata, which, like ripple-marks, seem, by themselves, quite insignificant. And among these he lays special emphasis upon those which show that during their deposition strata have at intervals been laid bare to sun and air.

During ebb tide water which has been left at the upper edge of the beach runs down across the beach in small rills, which excavate miniature channels; and when these are preserved in the hard rocks, they prove that the latter are beach deposits, and, like the ripple-marks, show the direction of the old shore.

If a heavy shower of rain falls on a muddy beach or flat, the sediment deposited by the returning tide may cover, without obliterating, the small but characteristic impressions of the individual drops; and these markings are frequently found well preserved in the hardest slates and sandstones, testifying unequivocally to the conditions under which the rocks were formed. In some cases the rain-prints are found to be ridged up on one side only, in such a manner as to indicate that the drops as they fell were driven aslant by the wind. The prominent side of the marking, therefore, indicates the side towards which the wind blew.

Muddy sediments, especially in lakes and rivers, are often exposed to the air and sun during periods of drouth, and as they gradually dry up, polygonal cracks are formed. The sediment of the next layer will fill these sun-cracks; and when, as often happens, it is slightly different from the dessicated layer, they may still be traced. Sun-cracks preserved in this way are very characteristic of argillaceous rocks, and, of course, prove that in early times, as at the present day, sediments of this class were exposed by the temporary retreat of the water. The foot-prints or trails of land-animals are often, as in the sandstones and shales of the Connecticut Valley, associated with, and of course strongly corroborate, all these other evidences of shore deposits. From the foot-prints preserved in the rocks we pass naturally to the consideration of the fossil remains of plants and animals found entombed in the strata.

FOSSILS.—Although fossils find their highest interest in the light which they throw upon the succession of life on the globe, they may also be properly regarded as structural features of stratified rocks; and any one who has seen the dead shells, crabs, fishes, etc., on the beach will readily understand how fossils get into the rocks. It is not our province here to study the structure of the fossils themselves, for that would involve us in a course in paleontology, a task belonging to the biologist rather than the geologist; but we will merely observe the three principal degrees in the preservation of fossils:—

1. _Original composition not completely changed._—Extinct elephants have been found frozen in the river-bluffs of Siberia so perfectly preserved that dogs and wolves ate their flesh. The bodies of animals are also found well preserved in peat-bogs. All coal is simply fossil vegetation retaining in a large degree the original composition; and the same is true of ferns, etc., preserved as black impressions in the rocks. All bones and shells consist of mineral matter which makes them hard, and animal matter which makes them tough and strong. In very many cases, especially in the newer formations, the animal matter is still partially, and the mineral matter almost wholly, intact.

2. _Original composition completely changed, but form and structure preserved._—All kinds of fossils are commonly called petrifactions, but only those preserved in this second way are truly petrified, _i.e._, turned to stone. “Petrified wood is the best illustration, and in a good specimen not only the external form of the wood, not only its general structure—bark, wood, radiating silver-grain, and concentric rings of growth—are discernible, but even the microscopic cellular structure of the wood, and the exquisite sculpturing of the cell-walls, are perfectly preserved, so that the kind of wood may often be determined by the microscope with the utmost certainty. Yet not one particle of the organic matter of the wood remains. It has been entirely replaced by mineral matter; usually by some form of silica. The same is true of the shells and bones of animals.”—LE CONTE.

3. _Original composition and structure both obliterated, and form alone preserved._—This occurs most commonly with shells, although fossil trees are also often good illustrations. The general result is accomplished in several ways: (_a_) The shell after being buried in the sediment may be removed by solution, leaving a _mould_ of its external form, (_b_) This mould may subsequently be filled by the infiltration of finer sediment, forming a _cast_ of the exterior of the shell. (_c_) The shell, before its solution, may have been filled with mud; and if the shell itself is then dissolved, we have a cast of its interior in a mould of its exterior.

TIME REQUIRED FOR THE FORMATION OF STRATIFIED ROCKS.—Many attempts have been made to determine the time required for the deposition of any given thickness of stratified rocks. Of course, only roughly approximate results can be hoped for in most cases; but these are at least sufficient to make it certain that geological time is very long. The average relative rate of growth of different kinds of sediment is, however, less open to doubt, for we have already seen that coarse sediments like gravel and sand accumulate much more rapidly than finer sediments like clay and limestone; and we are sometimes able to compare these two classes of rocks on a very large scale.

Thus, during what is known as the Paleozoic era, a sea extended from the Blue Ridge to the Rocky Mountains. Along the eastern margin of this sea, where the Alleghany Mountains now stand, sediments—chiefly conglomerate and sandstone, with some slate and less limestone—accumulated to a thickness of nearly 40,000 feet. Toward the west, away from the old shore-line, the coarse sediments gradually die out, and the formations become finer and thinner. In western Ohio and Indiana, slate and limestone predominate; while in the central part of the ancient sea, in Illinois and Missouri, the paleozoic sediments are almost wholly limestones, and have a thickness of only 4000 to 5000 feet. In other words, while one foot of limestone was forming in the Mississippi Valley, eight to ten feet of coarser sediments were deposited in Pennsylvania.

The best estimates show that coral-reefs rise—_i.e._, limestones are formed on them—at the rate of about one foot in two hundred years. But coral limestones grow much more rapidly than limestones in general. Sandstones sometimes accumulate so rapidly that trees are buried before they have time to decay and fall (Fig. 9). Such a buried forest, like a coal-bed, represents a land surface, and proves a subsidence of the land; and in some cases, as indicated by the section, repeated oscillations of the crust may be proved in this way.

The mud deposited by the annual overflow of the Nile is forty feet thick near the ancient city of Memphis; and the pedestal of the statue of Rameses II., believed to have been erected B.C. 1361, is buried to a depth of nine feet, four inches, indicating that 13,500 years have elapsed since the Nile began to spread its mud over the sands of the desert.

But the greatest difficulty in estimating the time required for the formation of any series of strata arises from the fact that we cannot usually even guess at the length of the periods when the deposition has been partially or wholly interrupted. Now and then, however, we find evidence that these periods may be very long. A layer of fossil shells in sandstone or slate proves an interruption of mechanical deposition. Beds of coal, fossil forests, and other indications of land surfaces are still more conclusive. The interposition of strata (Fig. 5) proves a prolonged interruption of deposition over the area not covered by the interposed bed. But the most important of all evidence is that afforded by unconformability; and the length of the lost interval between the two formations is measured approximately by the erosion of the older.

Original Structures of Eruptive Rocks.

The structures of this class are divisible into those pertaining to the volcanic rocks and those pertaining to the fissure or dike rocks. But since volcanoes are rare in this part of the world, while dikes are well developed in many sections of our country, it seems best to give our attention chiefly to the latter.

The term _dike_ is a general name for all masses of eruptive rocks that have cooled and solidified in fissures or cavities in the earth’s crust. But the name is commonly restricted to the more regular, wall-like masses (Fig. 10), those having extremely irregular outlines, like most masses of granite (Fig. 11), being known simply as _eruptive masses_. The propriety of this distinction is apparent when we consider the origin of _dike_ as a geological term. It was first used in this sense in southern Scotland, where almost any kind of a wall or barrier is called a dike. The dikes traverse the different stratified formations like gigantic walls, which are often encountered by the coal-miners, and on the surface are frequently left in relief by the erosion of the softer enclosing rock, so that in the west of Scotland, especially, they are actually made use of for enclosures. In other cases the dike has decayed faster than the enclosing rock, and its position is marked by a ditch-like depression. The narrow, straight, and perpendicular clefts or chasms observed on many coasts are usually due to the removal of the wall-like dikes by the action of the waves. Dikes are sometimes mere plates of rock, traceable for a few yards only; and they range in size from that up to those a hundred feet or more in width, and traceable for scores of miles across the country, their outcrops forming prominent ridges. The sides of dikes are often as parallel and straight of those of built walls, the resemblance to human workmanship being heightened by the numerous joints which, intersecting each other along the face of a dike, remind us of well-fitted masonry.

FORMS OF DIKES.—A dike is essentially a casting. Melted rock is forced up from the heated interior into a cavity or crack in the earth’s crust, cools and solidifies there, and, like a metallic casting, assumes the form of the fissure or mould. In other words, the form of the dike is exactly that of the fissure into which the lava was injected. Now the forms of fissures depend partly upon the nature of the force that produces them, but very largely upon the structure—and especially the joint-structure—of the enclosing rocks. Nearly all rocks are traversed by planes of division or cracks called joints, which usually run in several directions, dividing the rock into blocks. And it is probable that dike-fissures are most commonly produced, not by breaking the rocks anew, but by widening or opening the pre-existing joint-cracks. Hence the straight and regular jointing of slate, limestone and most sedimentary rocks is accompanied by wall-like dikes—the typical dikes (Fig. 10); while the more irregular jointing of granite and other massive rocks gives rise to sinuous, branching, variable dikes. The general dependence of dikes upon the joint-structure of the rocks is proved by the facts that dikes, like joints, are normally vertical or highly inclined, and that they are usually parallel with the principal systems of joints in the same district. The wall-like dikes also give off branches, but usually in a regular manner, as shown in Fig. 12.

STRUCTURE OF DIKES.—The rock traversed by a dike is called the _country_ or _wall_ rock. Fragments of this are often torn off by the igneous material, and become enclosed in the latter. Such enclosed fragments may sometimes form the main part of the dike, which then, since they are necessarily angular, often assumes the aspect of a breccia. This is the only important exception to the rule that dikes are homogeneous in composition; _i.e._, in the same dike we can usually find—from end to end, from side to side, and probably from top to bottom—no essential difference in composition. But there is often a marked contrast in _texture_ between different parts of a dike, and especially between the sides and central portion. The liquid rock loses heat most rapidly where it is in contact with the cold walls of the fissure, and solidifies before it has time to crystallize, remaining compact and sometimes even glassy; while in the middle of the dike, unless it is very narrow, it cools so slowly as to develop a distinctly crystalline texture. There is no abrupt change in texture, but a gradual passage from the compact border to the coarsely crystalline or porphyritic middle portion. It is obvious that a similar gradation in texture must exist between the top and bottom of a dike.

CONTACT PHENOMENA.—Under this head are grouped the interesting and important phenomena observable along the contact between the dike and wall-rock. These throw light upon the conditions of formation of dikes, and are often depended upon to show whether a rock mass is a dike or not. The student will observe here:—

1. The detailed form of the contact. It may be straight and simple, or exceedingly irregular, the dike penetrating the wall, and enclosing fragments of it, as in Fig. 11, which is a typically igneous contact.

2. The alteration of the wall-rock by heat. This may consist in: (_a_) _coloration_, shales and sandstones being reddened in the same way as when clay is burnt for bricks; (_b_) _baking and induration_, sandstone being converted into quartzite and even jasper; clay, slate, etc., being not only baked to a flinty hardness, but actually vitrified, as in porcelainite; and bituminous coal being converted into natural coke or anthracite; and (_c_) _crystallization_, chalk, and other limestones being changed to marble, and crystals of pyrite, calcite, quartz, etc., being developed in slate, sandstone, and other rocks.

3. The alteration of the dike-rock by (_a_) more rapid cooling, and (_b_) the access of thermal waters.

The alteration of the wall-rock may extend only a few inches or many yards from the dike, gradually diminishing with the distance; and the cases are surprisingly numerous where there is no perceptible alteration; and, again, the alteration is usually mutual, the dike-rock being altered in texture, color, and composition.

INTRUSIVE BEDS.—We commonly think of dikes as cutting across the strata, but they often lie in planes parallel with them; and the same dike may run across the beds in some parts of its course and between them in others (Fig. 12), or the conformable dike maybe simply a lateral branch of a main vertical dike, as shown in the same figure. All dikes or portions of dikes lying conformably between the strata are called _intrusive beds_ or _sheets_.

When a dike fails to reach the surface, but spreads out horizontally between the strata, forming a thick dome or oven-shaped intrusive bed, the latter is called a _laccolite_ (Fig. 13). Laccolites are sometimes of immense volume, containing several cubic miles of rock. Fig. 14 enables us to compare the laccolite with the volcano.

In the one case a large mound of eruptive material accumulates between the strata, the overlying beds being lifted into a dome; while in the other case the fissure or vent reaches the surface, and the mound of lava is built up on top of the ground.

COTEMPORANEOUS BEDS.—When the lava emitted by a crater is sufficiently liquid, it spreads out horizontally, forming a volcanic sheet or bed. If such an eruption is submarine, or the lava flow is subsequently covered by the sea, sedimentary deposits are formed over it; and beds of lava which thus come to lie conformably between sedimentary strata are known as _cotemporaneous sheets_ or _beds_, because they belong, in order of time, in the position in which we find them, being, like any member of a stratified series, newer than the underlying and older than the overlying strata. Cotemporaneous lava-flows are sometimes repeated again and again in the same district, and thus important formations are built up of alternating igneous and aqueous deposits. Evidently, the student who would read correctly the record of igneous activity in the past must be able to distinguish intrusive and cotemporaneous beds. The principal points to be considered in making this distinction are: (1) The intrusive bed is essentially a dike, dense and more or less crystalline in texture, altering, and often enclosing fragments of, both the underlying and overlying strata, and frequently jogging across or penetrating the sediments. (2) The cotemporaneous bed, on the other hand, being essentially a lava-flow, is much less dense and crystalline, being usually distinctly scoriaceous or amygdaloidal, especially at the borders, and the underlying strata alone showing heat action, or occurring as enclosures in the lava; for the overlying strata are newer than the lava, and often consist largely, at the base, of water-worn fragments of the lava.

AGES OF DIKES.—The ages of dikes may be estimated in several ways. They are necessarily newer than any stratified formation which they intersect or of which they enclose fragments; but any formation crossing the top of a dike must usually be regarded as newer than the dike, especially if it contains water-worn fragments of the dike rock.

The relative ages of different dikes are determined by their relations to the stratified formations; and still more easily by their mutual intersections, on the principle that when two dikes cross each other, the intersecting must be newer than the intersected dike. It is sometimes possible, in this way, to prove several distinct periods of eruption in the same limited district. The textures of dikes also often afford reliable indications of their ages; for, as we have already seen, the upper part of a dike, cooling rapidly and under little pressure, must be less dense and crystalline than the deep-seated portion, which cools slowly and under great pressure.

Now, the lower, coarsely crystalline part of a dike can usually be exposed on the surface only as the result of enormous erosion; and erosion is a slow process, requiring vast periods of time. Hence, when we see a coarse-grained dike outcropping on the surface, we are justified in regarding it as very old, for all the fine-grained upper part has been gradually worn away by the action of the rain, frost, etc. Other things being equal, coarse-grained must be older than fine-grained dikes; and the texture of a dike is at once a measure of its age and of the amount of erosion which the region has suffered since it was formed.

ERUPTIVE MASSES.—In striking contrast with the more or less wall-like dikes are the highly irregular, and even ragged, outlines of the eruptive masses; and it is worth while to notice the probable cause of this contrast. The true dikes are formed, for the most part, of comparatively fine-grained rocks—the typical “traps”; while the eruptive masses consist chiefly of the coarse-grained or granitic varieties. Now we have just seen that the coarse-grained rocks have been formed at great depths in the earth’s crust, while the fine-grained are comparatively superficial. But we have good reason for believing that the joint-structure, upon which the forms of dikes so largely depend, is not well developed at great depths, where the rocks are toughened, if not softened, by the high temperature. In other words, trap dikes are formed in the jointed formations, which break regularly; while the granitic masses are formed where the absence of joint-structure and a high temperature combine to cause extremely irregular rifts and cavities when the crust is broken.

VOLCANIC PIPES OR NECKS.—Every volcano and every lava-flow or volcanic sheet must be connected with the earth’s interior by a channel or fissure, which becomes a dike when the lava ceases to flow. But the converse proposition is not true, for it is probable that many dikes did not originally reach the surface, but have been exposed by subsequent denudation. This is conspicuously the case with laccolites and other forms of intrusive sheets. Volcanic sheets or beds have probably often resulted from the overflow of the lava at all points of an extensive fissure or system of fissures; but the vent of the true volcano must be more circumscribed, an approximately circular opening in the earth’s crust, although doubtless originating in a fissure or at the intersection of two or more fissures, the lava continuing to flow at the widest part of the wound in the crust long after it has congealed in the narrower parts. Such a tube is known as the neck or pipe of the volcano; and volcanic necks are a highly interesting class of dikes, since they determine the exact location of many an ancient volcano, where the volcanic pile itself has long since been swept away. Necks and dikes are the downward prolongations or roots of the volcanic cone or sheet, and cannot be exposed on the surface until the volcanic fires have gone out and the agents of erosion have removed the greater part of the ejected materials.

Hence, equally with the dikes which originally failed to reach the surface, they, wherever open to our observation, testify to extensive erosion and a vast antiquity.

Original Structures of Vein Rocks.

Many things called veins are improperly so called, such as dikes of granite and trap, and beds of coal and iron-ore. The smaller, more irregular, branching dikes, especially, are very commonly called veins, and to distinguish the true veins from these eruptive masses, they are designated as _mineral veins_ or _lodes_, although the term _lode_ is usually restricted to the metalliferous veins.

ORIGIN OF VEINS.—Various theories of the formation of veins have been proposed, but the most of these are of historic interest merely, for geologists are now well agreed that nearly all true veins have been formed by the deposition of minerals from solution in fissures or cavities in the earth’s crust. In many cases, especially where the veins are of limited extent, it seems probable that a part or all of the mineral matter was derived from the immediately enclosing rocks, being dissolved out by percolating water; and these are known as segregation or lateral secretion veins. But it is quite certain that as a general rule the mineral solutions have come chiefly from below, the deep-seated thermal waters welling up through any channel opened to them, and gradually depositing the dissolved minerals on the walls of the fissure as the temperature and pressure are diminished. This case, however, differs from the first only in deriving the vein-forming minerals from more remote and deeper portions of the enclosing rocks; and thus we see that vein-formation, whether on a large or a small scale, is always essentially a process of segregation.

We know that every volcano and every lava flow must be connected below the surface with a dike; and it is almost equally certain that the waters of mineral springs forming tufaceous mineral deposits on the surface, as in the geyser districts, also deposit a portion of the dissolved minerals on the walls of the subterranean channels, which are thus being gradually filled up and converted into mineral veins, which will be exposed on the surface when erosion has removed the tufaceous overflow. This connection of vein-formation with the superficial deposits of existing springs has been clearly proved in several important instances in Nevada and California.

Veins occur chiefly in old, metamorphic, and highly disturbed formations, where there is abundant evidence of the former existence of profound fissures, and in regions similar to those in which thermal springs occur to-day.

In the supplement to the lithological section the student will find the formation of a typical vein briefly described and contrasted with that of a typical dike; also a brief account of the lithological peculiarities of vein rocks, and general statements concerning their relative abundance and vast economic importance.

EXTERNAL CHARACTERISTICS OF VEINS.—The typical vein may be described as a fissure of indefinite length and depth, filled with mineral substances deposited from solution. Externally, it is very similar to the typical dike, for the fissures are made in the same way for both. Veins are normally highly inclined to the horizon; they exhibit in nearly every respect the same general relations to the structure of the country rock as dikes; and the ages of veins are determined in the same way as the ages of dikes.

The extensive mining operations to which veins have been subjected in all parts of the world, have made our knowledge of their forms below the surface very full and accurate. It has been learned in this way that very often the corresponding portions of the walls of a vein do not coincide in position, but one side is higher or lower than the other, showing that the walls slipped over each other when the fissure was formed or subsequently; and this faulting or displacement of the walls appears to be much more common with veins than with dikes, perhaps because the fissures remained open much longer. This slipping of the walls is the principal cause of the almost constant changes in the width of veins. For, since the walls are never true planes, and are often highly irregular any unequal movements must bring them nearer together at some points than at others. As a rule, the enormous friction accompanying the faulting, either crushes the wall-rock, or polishes and striates it, producing the highly characteristic surfaces known as _slicken-sides_. Where the wall is finely pulverized in this way, or is partially decomposed before or after the filling of the fissure, a thin layer of soft, argillaceous material is formed, separating the vein proper from the wall-rock. The miners call this the _selvage_; and it is a very characteristic feature of the true fissure veins.

Fragments of the wall-rock are frequently enclosed in veins, and the latter sometimes branch or divide in such a way as to surround a large mass of the wall, which is known as a “horse.” A similar result is accomplished when a fissure is re-opened after being filled, if the new fissure does not coincide exactly with the old. It has been proved that veins have thus been re-opened and filled several times in succession; and in this way fragments of the older vein material become enclosed in the newer.

Although usually determined in direction by the joint-structure of the country rock, veins are often parallel with the bedding, especially in highly inclined, schistose formations. Such interbedded veins are commonly distinctly lenticular in form, occupying rifts in the strata which thin out in all directions and are often very limited in extent.

Whether conforming with the joint-structure or bedding, veins are commonly arranged in systems by their parallelism, those of different systems or directions usually differing in age and composition, and the older veins being generally faulted or displaced when intersected by the newer.

INTERNAL CHARACTERISTICS OF VEINS.—Internally, veins and dikes are strongly contrasted; and it is upon the internal features, chiefly, as previously explained, that we must depend for their distinction. In metalliferous veins the minerals containing the metal sought for (the galenite, sphalerite, etc.) are the _ore_; while the non-metalliferous minerals (the quartz, feldspar, calcite, etc.) are called the _gangue_ or vein-stone proper. Although the combinations of minerals in veins are almost endless, yet certain associations of ores with each other and with different gangue minerals are tolerably constant, and constitute an important subject for the student of metallurgy and mining.

When a vein is composed of a single mineral, as quartz, it may rival a dike in its homogeneity. Most important veins, however, are composed of several or a large number of minerals, which may be sometimes more or less uniformly mixed with each other, but are usually distributed in the fissure in a very irregular manner. The great granite veins which are worked for mica, feldspar and quartz, are good illustrations, on a large scale, of the structure of veins in which several minerals have been deposited cotemporaneously. The individual minerals are found to a large extent, in great, irregular masses, with no order observable in their arrangement.

When a mineral is deposited from solution, it crystallizes by preference on a surface of similar composition, thus quartz on quartz, feldspar on feldspar, and so on; and it seems probable that this selective action of the wall-rock may be a principal cause of the irregular distribution of minerals in veins. It has often been observed in metalliferous veins that the richness varies with the nature of the adjacent country rock. This dependence of the contents of a fissure upon the wall-rock may be due in part to the selective deposition of the minerals, and in part to their derivation from the contiguous portions of the country or wall-rock, as in the so-called segregated veins. Temperature and pressure exert an important influence upon chemical precipitation, and it is, therefore, probable that the composition of many veins varies with the depth.

Frequently, perhaps usually, the minerals of composite veins are deposited in succession, instead of cotemporaneously, giving rise to the remarkable banded structure so characteristic of this class of veins. The first mineral deposited in the fissure forms a layer covering each wall, and is in turn covered by layers of the second mineral, and that by the third, and so on, until the fissure is filled, or the solution exhausted. The distinguishing features of this structure are shown in Fig. 15, in which _w w_ represents the wall-rock, _a a_, _b b_, _c c_ are successive layers of quartz, fluorite and barite, and the central band, _d_, is galenite. Since the vein grows from the outside inward, the outer layers are the oldest, and the central layers are the newest; again, the layers are symmetrically arranged, being repeated in the reverse order on opposite sides of the middle of the vein; and, lastly, in layers composed of prismatic crystals, as quartz (see the figure); the crystals are perpendicular to the wall and often project into, and even through, the succeeding layers. Such a crystalline layer is called a “_comb_” and the interlocking of the layers in this way is described as the _comb-structure_ of the vein. The banding of veins is thus strongly contrasted with stratification, and with the structure in dikes due to the more rapid cooling along the walls. The duplicate layers are often discontinuous and of unequal thickness, on account of the strong tendency to segregation in the materials. This is clearly shown in Fig. 16, drawn on a reduced scale from a polished section of a lead vein in Cumberland, England, contained in the Museum of the Boston Society of Natural History. In this the gangue minerals are fluorite (_f_) and barite (_b_). The central band (_f g_) is a darker fluorite containing irregular masses of galenite. The banded structure of veins is exactly reproduced in miniature in the banding of agates, geodes, and the amygdules formed in old lavas. Unfilled cavities frequently remain along the middle of the vein. When small, these are known as “pockets.” They are commonly lined with crystals; and when the latter are minute, the pockets are called druses. In metalliferous veins, the ore is much more abundant in some parts than in others, and these ore-bodies, especially when somewhat definite in outline, are known in their different forms and in different localities, as _courses_, _slants_, _shoots_, _chimneys_, and _bonanzas_ of ore. The intersections and junctions of veins are often among the richest parts, as if the meeting of dissimilar solutions had determined the precipitation of the ore.

Metalliferous veins, especially, are usually deeply decomposed along the outcrop by the action of atmospheric agencies. The ore is oxidized, and to a large extent removed by solution, leaving the quartz and other gangue minerals in a porous state, stained by oxides of iron, copper, and other metals, forming the _gossan_ or _blossom-rock_ of the vein.

PECULIAR TYPES OF VEINS.—In calcareous or limestone formations, especially, the joint-cracks and bedding-cracks are often widened through the solution of the rock by infiltrating water, and thus become the channels of a more or less extensive subterranean drainage, by which they are more rapidly enlarged to a system of galleries and chambers, and, in some cases, large limestone caverns. The water dripping into the cavern from the overlying limestone is highly charged with carbonate of lime, which is largely deposited on the ceiling and floor of the cavern, forming stalactitic and stalagmitic deposits. These are masses of mineral matter deposited from solution in cavities in the earth’s crust, and are essentially vein-formations. Portions of caverns deserted by the flowing streams by which they were excavated, are often filled up in this way, being converted into irregular veins of calcite. But calcite is not the only mineral found in these cavern deposits, for barite and fluorite, and various lead and zinc ores, especially the sulphides of these metals—galenite and sphalerite—have also been leached out of the surrounding limestone and concentrated in the caverns. The celebrated lead mines of the Mississippi Valley, and some of the richest silver-lead mines of Utah and Nevada are of this character. The forms of these cavern-deposits vary almost indefinitely, and are often highly irregular. The principal types are known as _gash-veins_, _flats_ and _sheets_ (Fig. 17), _chambers_ and _pockets_.

Where joints and other cracks have opened slightly in different directions and become filled with infiltrated ores, we have what the German miners call a _stock-work_,—an irregular network of small and interlacing veins.

An _impregnation_ is an irregular segregation of metalliferous minerals in the mass of some eruptive or crystalline rock. Its outlines are not sharply defined, but it shades off gradually into the enclosing rock.

_Fahlbands_ are similar ill-defined deposits or segregations in stratified rocks. An impregnation or vein occurring along the contact between two dissimilar rocks is called a _contact deposit_. These are usually found between formations of different geological ages, and especially between eruptive and sedimentary rocks.

Subsequent Structures produced by Subterranean Agencies.

The subterranean forces concerned in the formation of rocks are chiefly various manifestations of that enormous tangential pressure developed in the earth’s crust, partly by the cooling and shrinkage of its interior, but largely, it is probable, by the diminution of the velocity of the earth’s rotation by tidal friction, and the consequent diminution of the oblateness of its form. It is well known that the centrifugal force arising from the earth’s rotation is sufficient to change the otherwise spherical form of the earth to an oblate spheroid, with a difference of twenty-six miles between the equatorial and polar diameters. It is also well known that while the earth turns from west to east on its axis, the tidal wave moves around the globe from east to west, thus acting like a powerful friction-brake to stop the earth’s rotation. Our day is consequently lengthening, and the earth’s form as gradually approaching the perfect sphere. This means a very decided shortening and consequent crumpling of the equatorial circumference, and is equivalent to a marked shrinkage of the earth’s interior, so far as the equatorial regions are concerned.

The most important and direct result of the horizontal thrust, whether due to cooling or tidal friction, is the corrugation or wrinkling of the crust; and the earth-wrinkles are of three orders of magnitude,—continents, mountain-ranges, and rock-folds or arches.

Continents and ocean-basins, although the most important and permanent structural features of the earth’s crust, do not demand further consideration here, since their forms and relations are adequately described in the better text-books of physical geography. The forms and distribution of mountain-ranges might be dismissed in the same way; but, unlike continents, the structure of mountains, upon which their reliefs mainly depend, is quite fully exposed to our observation, and is one of the most important fields of the student of structural geology. Mountains, however, as previously explained, combine nearly all the kinds of structure produced by the subterranean agencies, and their consideration, therefore, belongs at the end rather than the beginning of this section.

INCLINED OR FOLDED STRATA.—Normally, strata are horizontal, and dikes and veins are vertical or nearly so. Hence the stratified rocks are more exposed to the crumpling action of the tangential pressure in the earth’s crust than the eruptive and vein rocks; and it is for this reason and partly because the stratified rocks are vastly more abundant than the other kinds, that the effects of the corrugation of the crust are studied chiefly in the former. But it should be understood that folded dikes and veins are not uncommon.

That the stratified rocks have, in many instances, suffered great disturbance subsequent to their deposition, is very evident; for, while the strata must have been originally approximately straight and horizontal, they are now often curved, or sharply bent and contorted, and highly inclined or even vertical. All inclined beds or strata are portions of great folds or arches. Thus we may feel sure when we see a stratum sloping downward into the ground, that its inclination or dip does not continue at the same angle, but that at some moderate depth it gradually changes and the bed rises to the surface again. Similarly, if we look in the opposite direction and think of the bed as sloping upward—we know that the surface of the ground is being constantly lowered by erosion, and consequently that the inclined stratum formerly extended higher than it does now, but not indefinitely higher; for, in imagination, we see it curving and descending to the level of the present surface again. Hence it forms, at the same time, part of one side of a great concave arch, and of a great convex arch, just as every inclined surface on the ground indicates both a hill and a valley. And guided by this principle we can often reconstruct with much probability folds that have been more or less completely swept away by erosion, or that are buried beyond our sight in the earth’s crust.

The arches of the strata are rarely distinctly indicated in the topography, but must be studied where the ground has been partly dissected, as in cliffs, gorges, quarries, etc. They are also, as a rule, far more irregular and complex than they are usually conceived or represented. The wrinkles of our clothing are often better illustrations of rock-folds than the models and diagrams used for that purpose. This becomes self-evident when we reflect that the earth’s crust is exceedingly heterogeneous in composition and structure, and must, therefore, yield unequally to the unequal strains imposed upon it.

The folds or undulations of the strata may be profitably compared with water-waves. In fact, the comparison is so close that they have been not inaptly called rock-waves. Folds, like waves, unless very large, rarely continue for any great distance, but die out and are replaced by others, giving rise to the _en echelon_ or step-like arrangement. The plan of both a wave and a fold is a more or less elongated ellipse, each stratum in a fold being semi-ellipsoidal or boat-shaped. In other words, a normal fold is an elongated mound of concentric strata, being highest at the centre, sloping very gradually toward the ends, and much more abruptly toward the sides.

The imaginary line passing longitudinally through a fold, about which the strata appear to be bent, is the _axis_; and the plane lying midway between the two sides of a fold and including the axis is the _axial plane_. The two principal kinds of folds are the _anticline_ (Fig. 18, _A_), where the strata dip away from the axis; and the _syncline_ (Fig. 18, _B_), where they dip toward the axis. They are commonly, but not always, correlative, like hill and valley.

Rock-folds are of all sizes, from almost microscopic wrinkles to great arches miles in length and breadth, and thousands of feet in height. The smaller folds, or such as may be seen in hand specimens and even in considerable blocks of stone, are commonly called contortions, and it is interesting to observe that they are, in nearly everything except size, precisely like the large folds, so that they answer admirably as geological models. Large folds, however, are almost necessarily curves, but contortions are frequently angular (Fig. 19). With folds, as with waves, the small undulations are borne upon the large ones; but the contortions are not uniformly distributed. An inspection of Fig. 18 shows that when the rocks are folded they must be in a state of tension on the anticlines (_A_), and in a state of compression in the synclines (_B_), and the latter is evidently the normal position of the puckerings or contortions of the strata, as shown in Fig. 20. Contortions are also most commonly found in thin-bedded, flexible rocks, such as shales and schists. And when we find them in hard, rigid rocks, like gneiss and limestone, it must mean either that the structure was developed with extreme slowness, or that the rock was more flexible then and possibly plastic.

It is very interesting to notice the relations of anticlinal and synclinal folds to the agents of erosion. At the time the folds are made, the anticlinals, of course, are ridges, and the synclinals, valleys, and this relation sometimes continues, as shown in Fig. 21; but we have seen that the rocks in the trough of the synclinal are compressed and compacted, _i.e._, made more capable of resisting erosion, while those on the crest of the anticlinal are stretched and broken, _i.e._, made more susceptible of erosion. The consequence is that the anticlinals are usually worn away very much faster than the synclinals; so much faster that in many cases the topographic features are completely transposed, and in place of anticlinal ridges and synclinal valleys (Fig. 21) we find synclinal ridges and anticlinal valleys (Fig. 22).

Besides the anticlinal and synclinal folds already explained, there are folds that slope in only one direction, one-sided or _monoclinal_ folds (Fig. 23). Anticlinal and synclinal folds are _symmetrical_ when the dip or slope of the strata is the same on both sides and the axial plane is vertical. The great majority of folds, however, are _unsymmetrical_, the opposite slopes being unequal, and the axial planes inclined to the vertical (Fig. 24, _A_). This means that the compressing or plicating force has been greater from one side than from the other, as indicated by the arrows. It acted with the greatest intensity on the side of the gentler slope, the tendency evidently having been to crowd or tip the fold over in the direction of the steep slope. When the steep slope approaches the vertical, this tendency is almost unresisted, and when it passes the vertical, gravitation assists in overturning the fold (Fig. 24, _B_). Such highly unsymmetrical folds, including all cases where the two sides of the fold slope in the same direction, are described as _overturned_ or _inverted_, although the latter term is not strictly applicable to the entire fold, but only to the strata composing the under or lee side of it. Fig. 24, _B_, shows that these beds are completely inverted, the older, as the figures indicate, lying conformably upon the newer. This inversion is one of the most important features of folded strata, and it has led to many mistakes in determining their order of succession. In the great mountain-chains, especially, it is exhibited on the grandest scale, great groups of strata being folded over and over each other as we might fold carpets. An inverted stratum is like a flattened S or Z, and may be pierced by a vertical shaft three times, as has actually happened in some coal mines. Folds are _open_ when the sides are not parallel, and _closed_ when they are parallel, the former being represented by a half-open, and the latter by a closed, book. Closed folds are usually inverted, and when the tops have been removed by erosion (Fig. 25), the repetition of the strata may escape detection, and the thickness of the section be, in consequence, greatly overestimated. Thus, a geologist traversing the section in Fig. 25 would see thirty-two strata, all inclined to the left at the same angle, those on the right apparently passing below those on the left, and all forming part of one great fold. The repetition of the strata in reverse order, as indicated by the numbers, and the structure below the surface, show, however, that the section really consists of only four beds involved in a series of four closed folds, the true thickness of the beds in this section being only one-eighth as great as the apparent thickness.

The most important features to be noted in observing and describing inclined or folded strata are the _strike_ and _dip_. The strike is the compass bearing or horizontal direction of the strata. It is the direction of the outcrop of the strata where the ground is level. It may also be defined as the direction of a level line on the surface of a stratum, and is usually parallel with the axis of the fold.

The dip is the inclination of the beds to the plane of the horizon, and embraces two elements: (_a_) the direction of the dip, which is always at right angles to the strike, being the line of steepest descent on the surface of the stratum, and (_b_) the amount of the dip, which is the value of the angle between the line of steepest descent and the horizon.

In Fig. 26, _s t_ is the direction of the strike, and _d p_ that of the dip. The strike and direction of the dip are determined with the compass, and the amount of the dip with the clinometer, an instrument for measuring vertical angles.

The strike is much less variable than the dip, being often essentially constant over extensive districts; while the dip, except in very large or closed folds, is constantly changing in direction and amount.