Part 10
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.