The structures of rock divide, at the outset, into two classes:—(1) the _original structures_, or those produced at the same time and by the same forces as the rocks themselves, and which are, therefore, peculiar to the class of rocks in which they occur (_e.g._ stratification, ripple-marks, fossils, etc.); and (2) the _subsequent structures_, or those developed in rocks subsequently to their formation, and by forces that act more or less uniformly upon all classes of rocks, and which are, therefore, in a large degree, common to all kinds of rocks (_e.g._ folds, faults, joints, etc.).
The original structures are conveniently and naturally classified in accordance with the three great classes of rocks: (1) stratified rocks, (2) eruptive rocks, and (3) vein rocks; while the subsequent structures, not being peculiar to particular classes of rocks, are properly divided into those produced by (1) the subterranean or igneous agencies, and (2) the superficial or aqueous agencies.
[Illustration: Fig. 1.—Section through sediment deposited by rain in a roadside pool: _a._ surface of roadway; _b._ layer of small pebbles and coarse sand; _c._ fine sand passing into _d_; _d._ the finest sand and mud.]
STRATIFICATION.—All rocks formed by strewing materials in water, and their deposition in successive, parallel, horizontal layers, are _stratified_; and this structure is their _stratification_. It is the most important of all rock structures; and there is no kind of structure the origin of which is more fully or certainly known. The deposition of sediment in carefully assorted horizontal layers is readily brought within the comprehension of children by simple experiments with sand and clay in water; and still better by the examination of the deposits formed in roadside pools during heavy rains (Fig. 1), and by digging into beaches and sandbars, which every child will recognize as formed of materials arranged by water. Great stress should be laid upon the fact that a lake like Erie or Champlain is simply a large pool with several more or less turbid streams flowing into it, while the single stream flowing out is clear, the sediment having evidently been deposited in the lake; and that every lake is, like the roadside pool, being gradually filled up with sedimentary or stratified rocks. But the ocean is a still larger pool, receiving mud and sand from many streams; and since we know that nothing escapes from the ocean but invisible vapor, it is plain that the mud and sand and all other kinds of sediment carried into the ocean must be deposited on its floor, and chiefly, as we have seen, on that part nearest the land. The consolidation of beaches, bars, and mud-flats is all that is necessary to convert them into stratified formations of conglomerate sandstone and slate.