The structural phases which rocks assume.—In the previous chapters, the general method by which rocks are formed has been set forth, and many of their structural features have been touched upon incidentally. It remains to assemble the structural features already mentioned, and to consider certain additional structural phases which rocks assume.
In the deposition of sediments in the sea, or in other bodies of standing water, the coarser portion of the material is usually deposited in the shallow water near the shore where the wave-action is strongest, and the less coarse of various grades is deposited at greater and greater distances from the land, while only extremely fine silt is usually carried out to abysmal depths (see p. 380). To this general law of distribution there are important exceptions. Fine sediments are sometimes deposited near the shore, and where currents, tidal agitation, or floating ice are effective, coarse deposits are occasionally carried far out from the shore.
Stratification.—Sedimentary rocks are usually arranged in more or less distinct layers; that is, they are stratified. The stratification consists primarily in the superposition of layers of different constitution or different compactness on one another. Layers of like constitution or compactness are often separated by films of different material which cause the partings between them. The bedded arrangement of stratified rocks is due to various causes, but primarily to the varying agitation of the waters in which the sediment was laid down. Where the depositing waters are agitated to the bottom, coarse sediment is likely to be deposited. Where the waters are quiet at the bottom, fine sediment is the rule. Since the agitation of the waters is subject to frequent change, it follows that coarser material succeeds finer, and finer coarser, in the same place. Hence arise beds, layers, and laminæ. The terms layer and bed are generally used as synonyms, while laminæ are thinner divisions of the same sort. The term stratum is sometimes applied to one layer and sometimes to all the consecutive layers of the same sort of rock. For the latter meaning the term formation is often used. Sometimes bedding seems to have been determined by strong currents which temporarily not only prevented deposition over a given area, but even cut away the loose surface of deposits already made, giving a firm surface from which succeeding deposits are distinct. This sequence of events is sometimes shown by the truncation of laminæ, and by other signs of erosion. The commoner sorts of bedded rock are limestones, shales, sandstones, and conglomerates.
The bedding of limestones is often caused by the introduction of thin films of clayey material which interrupt the continuity of the lime accumulation and cause natural partings. Sometimes, however, bedding arises from variations in the physical condition of the lime sediment itself. Lamination is not usually conspicuous in pure limestone, though it may be well developed in the shaly phases of this rock. Shales are normally laminated as well as bedded, and the lamination is often more notable than the thicker bedding. Bedding in shale may arise from the introduction of sandy laminæ, or by notable changes in the texture of the shale material. Similarly, sandstones are sometimes divided into beds by shaly (clayey) partings, but more often by variations in the coarseness of the sand itself, or by the presence of laminæ that are less coherent than those above and below. Sometimes the layers appear to be determined by the compacting of the surface of sand already accumulated before it was buried by later deposits. Sandstones may be thick- or thin-bedded, and their bedding passes insensibly into lamination.
Sand deposits usually take place in relatively shallow water, and the sand is subjected to much shifting before it finds a permanent lodgment. In the course of this shifting, bars are formed which usually have a rather steep face in the direction in which they are being shifted. The sand carried over the top of the bar finds lodgment on the sloping terrace face. The inclined laminæ thus formed constitute a kind of bedding, but since its planes do not conform to the general horizontal attitude of the formation as a whole, it is called false- or cross-bedding or, more accurately, cross-lamination (see Fig. 368). The same structure is developed on delta fronts and generally in water shallow enough to be subject to frequent agitation at the bottom. Sandstone is cross-bedded more commonly than other sorts of sedimentary rock.
The bedding of conglomerate is due chiefly to variations in coarseness. Laminæ or thicker layers of sand are frequently found between layers of coarser material. Conglomerate is likely to be thick-bedded, and cross-bedding is common.
Lateral gradation.—When the varying nature of the agitation of the sea at different depths and along the different parts of the coast-border, and during different phases of the sea-currents, is considered, it will be readily understood that sedimentary beds are affected by many irregularities, and that deposits of one kind grade into others horizontally with great freedom. Thus a bed of conglomerate (gravel) may grade laterally into sandstone, and this into shale or limestone. It is indeed rather more remarkable that the sedimentary strata should be as regular and persistent as they are, than that they sometimes grade into one another.
Special markings.—The rhythmical action of waves gives rise to undulatory lodgment, known as ripple-marks (Fig. 324). They are usually not the direct product of the surface-waves, since they are much too small. They are produced mainly by the vibratory movement of the undertow, but they apparently result from various other phases of vibratory agitation of the bottom waters. They are sometimes made by streams and stream-like currents. Ripple-marks are apparently preserved indefinitely under proper circumstances. They are sometimes found, for example, on very ancient quartzites. Ripples are also made by wind (p. 37). Ripple-marks are usually only an inch or two from crest to crest, but in rare instances they attain much greater size. Examples of ripple-marks 30 feet across are known.[211] Occasional ridges and depressions of much greater dimensions are produced which are attributable to the formation of successive bars, or to the building of wave-cusps.[212] Rill-marks are not infrequently produced by the undertow and other currents passing over pebbles, shells, etc. (Figs. 325 and 326).
Sediments are sometimes exposed between tides, or under other circumstances, for periods long enough to permit drying and cracking at the surface. On the return of the waters, the cracks may be filled and permanently preserved. These are known as sun-cracks or mud-cracks (Figs. 328 and 369). They chiefly affect shales, but are occasionally seen in limestones and fine-grained sandstones. During the exposure of the sediments a shower may pass and rain-drop impressions (Fig. 370) be made which are subsequently filled by fine sediment and preserved. The size and depth of rain-drop impressions give some hint as to the meteorological conditions of far-off ages. Wave-marks, which consist of the faint line-ridges developed on a sandy beach at the limit of the incoming wave, are sometimes preserved and may be seen occasionally on layers of rock deposited millions of years ago.
Concretionary structure.—Various sedimentary formations contain nodules or irregularly shaped masses of mineral matter unlike the rock in which they occur. When these nodules consist of matter aggregated about some center, they are called concretions. They are common in sedimentary rocks, and here it may sometimes be seen that the aggregation has taken place about a shell, a leaf, or some other organic relic. The nuclei are, however, not always organic. The material of the concretion may have come from the immediately surrounding rock, having been first dissolved by water and then deposited about the nucleus, or it may have been introduced from without, likewise by the agency of water. In the first case, the mineral matter of the concretion is usually one of the minor constituents of the rock. Thus the commonest concretions in limestone are composed of impure silica (chert, Fig. 361); in shale, of lime carbonate or iron sulphide; in sandstone, of iron oxide. The concretion may be made up almost wholly of concentrated matter, in which case the matter originally in the place of the concretion has been crowded aside; or it may involve much of the material of the imbedding rock. Thus the concretion of lime carbonate in shale may be nearly pure, or it may involve much of the earthy matter of the shale, while the concretion of iron oxide in sandstone commonly includes much sand. In extreme cases, indeed, the concentrated matter of the concretion merely cements the material involved into distinct nodules. Occasionally the rock substance itself takes on a concretionary form, all or most of its material being involved.
In size, concretions may vary from microscopic dimensions to huge masses, 8, 10, or even more feet in diameter. The variations in shape are also great. They may be spherical, elliptical, discoid, or they may assume more irregular and complex forms (Figs. 371 and 372). The conditions of growth have much to do with the form. Thus a concretion which starts as a sphere may find growth easier in one plane than another, when it becomes discoid. Two or more concretions sometimes grow together, giving rise to complicated forms. Some of the most complex and fantastic forms are perhaps to be explained in this way. Concretions sometimes take the form of tubes. Some minute tubular concretions were formed about rootlets, but the larger ones appear to owe their form to other influences (Fig. 374).
One of the most extraordinary features of some concretions of complex form is their symmetry. This may be of various phases; in exceptional cases there is a bilateral symmetry almost as perfect as in the higher types of animals. This is especially true of certain calcareous concretions developed in plastic clays (Fig. 373).
Concretions sometimes develop cracks within themselves, and these may then be filled with mineral matter differing in composition or color from that of the original concretions (Figs. 375 and 376). Concretions the cracks of which have been filled by deposition from solution, are called septaria. They are especially abundant in some of the Cretaceous shales and clays. In not a few cases the filling of the cracks appears to have wedged segments of the original concretion farther and farther apart, until the outer surface of the septarium is made up more largely of vein-matter than of the original concretion (Fig. 377). Such concretions are often popularly known as “petrified turtles.”
Concretions of the sort indicated above often develop after the enclosing sedimentary rock was deposited. This is shown, among other things, by the fact that numerous planes of lamination may sometimes be traced through the concretions.
Concretions also form in water during the deposition of sedimentary rock. Exceptionally, sedimentary rock is made up chiefly of concretions. The chemical precipitates from the concentrated waters of certain enclosed lakes sometimes take the form of minute spherules which resemble the roe of fish. From this resemblance the resulting rock is called oolite (Fig. 357). Oolite is now forming about some coral reefs, presumably from the precipitation of the lime carbonate which was temporarily in solution. Considerable beds of limestone are sometimes oolitic. The calcium carbonate of such rock may be subsequently replaced by silica, so that the oolitic structure is sometimes found in silicious rock. If the concretions become larger, say as large as peas, the rock is called pisolite instead of oolite (Fig. 378).
Beds of iron ore are likewise sometimes concretionary. Thus in the Clinton formation there are widespread beds of “flaxseed” ore made up of concretions of iron oxide which, individually, resemble the seed which has given the ore its name. The nucleus in this case is usually too small for identification.
Secretions.—When cavities in rock are filled by material deposited from solution, the result is sometimes called a secretion. Secretions therefore grow from without toward a center, while concretions follow the opposite order. Crystal-lined cavities (geodes, Fig. 359) and agates (Fig. 358) are examples of secretions. Crystal-lined cavities and veins are the same in principle.
Certain structural features of igneous rocks have been mentioned in treating of their origin in the previous chapter. When a great flow of lava spreads out upon the surface, there is no internal lamination or stratification, and the resulting rock is usually classified as massive rather than stratified; but when a succession of flows occur, each individual flow forms a layer, and the series as a whole becomes stratiform. The successive flows are not usually coextensive. If the later flows of the closing stages of a period of vulcanism fail to reach as far as the earlier ones, a terraced or step-like aspect is given to the region, whence the name trap-rock (trappe, steps) is derived. Such lava sheets, especially if of basalt, often assume a columnar structure in cooling, the columns being rude six-sided prisms standing at right angles to the cooling surfaces (Figs. 379 and 380). This phenomenon is usually best developed where the sheet is intruded between layers of preexisting rock in the form of sills. The formation of the columns is sometimes regarded as a variety of concretionary action, but more commonly as a result of contraction. The former is suggested by the ball-and-socket ends of the sections of some columns (Fig. 382). The development of the columns by contraction may be explained as follows: The surface of the homogeneous lava contracts about equally in all directions on cooling. The contractile force may be thought of as centering about equidistant points. About a given point, the least number of cracks which will relieve the tension in all directions is three (Fig. 383). If these radiate symmetrically from the point, the angle between any two is 120°, the angle of the hexagonal prism. Similar radiating cracks from other centers complete the columns (Fig. 384). A five-sided column would arise from the failure of the cracks to develop about some one of the points (Fig. 385).
When lava is forced into crevices or rises to the surface through fissures, and the residual portion solidifies in them, it gives rise to dikes, as illustrated in Figs. 2 and 417 (not a true dike). Dikes are sometimes affected by columnar structure. In this case, as in all others, the columns are likely to be at right angles to the cooling surface. Lava solidifying in the passageway leading from the interior of a volcano gives rise to a neck or plug. If the lava is forced between beds of rock in the form of a sheet, and solidifies there, it is called a sill. If, after rising to a certain point in the strata, the lava arches the beds above into a dome, and forms a great lens-like or cistern-like mass, it constitutes a laccolith (Fig. 334). If an intrusion of the laccolithic type faults the overlying beds instead of arching them, and especially if the vertical dimension of the intruded mass be great in comparison with its lateral dimensions, its shape is more like that of a plug or core. Such an intruded core is a bysmalith[213] (Fig. 124). Between the bysmalith and the laccolith there are various gradations, just as between the laccolith and the sill. When lava forces aside the rocks at considerable depths or absorbs them by solution or by “stoping,” and then solidifies in great masses of irregular or undetermined forms, these masses are called batholiths.
Fig. 381.—Sections of columns from Giant’s Causeway, coast of Ireland.
Fig. 382.—Ball-and-socket joints in columns of basalt. (Scrope.)
Fig. 383.—Diagram to illustrate the first stages in the formation of hexagonal columns by contraction.
Fig. 384.—The completion of the hexagonal columns.
Fig. 385.—Diagram to illustrate the development of five-sided columns.
Volcanic cones are familiar structures built up about the vents of active volcanoes, and will be discussed under vulcanism.
Inclination and folding of strata.—The original attitude of beds, whether formed by water or by lava-flows, is normally horizontal, or nearly so. Both kinds of deposits, however, occasionally take place on considerable slopes. Modifications of the original attitude result from earth movements, and the measurement of these modifications is an important feature of field study. It is recorded in terms of dip and strike. The dip is the inclination of the beds referred to a horizontal plane, as illustrated in Fig. 386, and is usually measured by a clinometer, the principle of which is shown in Fig. 387. In measuring the dip, the maximum angle is always taken. In Fig. 386, for example, the angle would be less if the direction were either to the right or left of that indicated by the arrow. The direction as well as the amount of the dip is always to be noted. This must be determined by the compass, to which the clinometer may be conveniently attached. Dip 40°, S. 20° W. gives the full record of the position of the bed of rock under consideration. The strike is the direction of the horizontal edge of dipping beds, or more generally, the direction of a horizontal line on the surface of the beds. This is illustrated in Fig. 386. Since the strike is always at right angles to the dip, the strike need not be recorded if the direction of the dip is. Thus dip 40°, S. 20° W. is the same as dip 40°, strike N. 70° W.
When the beds incline in a single direction, they form a monocline. When beds are arched so as to incline away from one another, they form an up-fold or anticline (Figs. 388 to 391). The anticline may depart from its simple form, as shown in Figs. 390 and 391. When beds are curved downward so as to incline towards one another, they form a syncline (Fig. 392). When beds assume the position shown in Fig. 393, the folds are said to be isoclinal. When they are arched so as to form a cone or dome, and incline in all directions from a central point, they are said to have a quaquaversal dip. When considerable tracts are bent so as to form great arches or great troughs with many minor undulations on the flanks of the larger, they are designated as geanticlines, or anticlinoria (Figs. 394 and 395), and geosynclines or synclinoria (Figs. 396 and 397). Folding is often accompanied by the development of slaty cleavage (p. 440).
Fig. 399.—Diagram to show how dip and strike are recorded.
Fig. 400.—Map record of dip and strike, showing synclinal structure.
Fig. 401.—Map record of dip and strike showing anticlinal structure.
Fig. 402.—The structure of the area shown in Fig. 401, in cross-section.
As found in the field, folds are usually much eroded, and often completely truncated (Fig. 398). The determination of anticlinal or synclinal structure is then not based on topography, or even on such sections as shown in Figs. 394 to 397, for such sections are relatively rare. The structure is determined by a careful record of dips and strikes. On the field map, the record may be made as shown in Figs. 399 to 401, where the free ends of the lines with but one free end point in the direction of dip, while the other lines represent the directions of strike. Applying this method, the structure shown in Fig. 400 represents a syncline, and that in Fig. 401 an anticline. In cross-section, the structure presented by Fig. 401 would appear as in Fig. 402. Fig. 403 shows a doubly plunging anticline; that is, an anticline the axis of which dips down at either end. Fig. 404 shows a combination of synclines and anticlines, and Fig. 405 a cross-section along the line ab of Fig. 404. The outcrops of rock where the dip and strike may be determined may be few and far between, but when they are sufficiently near one another, the structure of the rock, as shown in Fig. 405, may be worked out, even though the surface be flat.
Much the larger portion of the earth’s surface is occupied by beds that retain nearly their original horizontal attitude; but in mountainous regions the beds have usually suffered bending, folding, crumpling, and crushing, in various degrees, in the course of the deformations that gave rise to the mountains. Distortion is on the whole most intense and characteristic in the most ancient rocks known, the Archean, in which a distorted condition is nearly universal, so far as observation goes. Distortion is assigned chiefly to lateral thrust arising from the shrinkage of the earth, as explained in the chapter on Earth Movements. The simpler, and some rather complex forms of deformation, are shown in the preceding figures, but the folding is sometimes much more complex (Fig. 406), the folds sometimes “fan” (Fig. 407), and the beds of which they are composed are sometimes intricately crumpled (Figs. 408 to 410). Among these various phases of deformation there are all gradations and combinations. Overturned folds reverse the order of the strata in the under limb of the fold. After such folds have been greatly eroded, so that their outer form is lost and their relations have become obscure, the reversed beds are likely to be interpreted as though they lay in natural order. In such a case as that represented in Fig. 411, a complex structure may be interpreted as a simple one. Thus the strata of Fig. 411 may have the structure shown in Fig. 412, 413, or 414, so far as dip and strike show.