STRUCTURAL GEOLOGY treats of the architecture of the earth's crust, beginning with stratification and its accompaniments; aqueous or rocks are arranged in layers or strata, the strata expressing their leading structural feature. The general aspects of stratification will be best followed in an explanation of the terms by they are expressed. LaillinCV are the thinnest paper-like layers of deposit in a stratified rock. These layers will generally split apart, but sometimes are so compact that the rock breaks the soonest. Larnince occur only where the material is fine grained, as in mud or shale. The existence of lamina points to tranquil conditions of slow intermit tent deposit. A great thickness of laminated rock, like the massive shales of paleo zoic formation, points to a prolonged period of quiescence, and probably in most cases to slow and tranquil subsidence of the sea-floor. Strata or beds are layers of rock from an inch or less up to many feet in thickness. A stratum may be made of many lamime, and this has commonly been the case where the sediment has been exceedingly fine grained. Where the materials are of coarser grain, the strata, as a rule, are not lami nated, but form the thinnest parallel divisions of the mass of rock. Strata are usually, with more or less ease, separable from each other. A stratum may be one of a series of similar beds in the same mass of rock, or may be complete and distinct in itself, as where one of limestone or iron-stone runs through the heart of a series of shales. As a general rule we may conclude that wherever, among sedimentary accumulations, stratification is exceedingly well marked, the rocks were formed rather slowly; and that where it is weak or absent, the conditions of deposit were more rapid, without the intervals and changes necessary for the production of the distinctly stratified structure. is where some strata, especially sandstones, are marked by an irregular lamination, wherein the laminte, though for short distances parallel to each other, are oblique to the general stratification of the mass, at constantly varying angles and in different directions. Such a structure indicates frequent changes in the direction of the currents by which the sediment was carried along and deposited. Irregularities of bedding, due to inequalities of deposition or of erosion, indicate that a ridge of sand or gravel is-laid down under water by current action of some strength.
Should the motion of the water diminish, finer sediment may be brought to the place, and be deposited around and above the ridge. In such a case the stratification of the latter accumulation will end off abruptly against the flanks of the older ridge, which will appear to rise up through the overlying bed. Appearances of this kind are not uncommon in some coal-fields, where they are known to miners as "rolls," "swells," or " horses' backs." A structure exactly the reverse occurs where a stratification has been scooped out before the deposition of the layers which cover it. Such channels have evidently been water-courses, worn out of the coal-measure strata, at a compara tively recent geological period, and subsequently buried under glacial accumulations. There is a complete discordance between them and the paleozoic strata below, pointing to the existence of a vast interval of time. in sandstone are wavy ridges and hollows, such as may be seen on any shore from which the tide has retired. Their general direction suffices to indicate the quarter whence the chief movement of the water has come. Such indications of shallow-water conditions may often be observed among old crenaceous deposits, as in the Cambrian and Silurian rocks. tines, etc., prove that during deposition aqueous strata have been laid bare to the air and the sun. The nature and validity of this evidence will be best ascertained by observations made at the margin of the sea, or of any inland sheet of water, which, from time to time, leaves tracts of mud or fine sand exposed to sun and rain. Their existence in any strata proves that the surface of the rock on which they lie was exposed to the air and dried before the next layer of water-borne sediment was deposited upon it. Prints of rain-drops are often associated with these marks, and these serve some times to show from which direction the wind was blowing when the rain fell. Proofs of shallow shore-water, and of exposure to the air, are supplied by markings left by animals. Voidings and trails of worms, tracks of mollusks and crustaceans, fin-marks of fishes, footprints of birds, reptiles, and mammals, may all be preserved and give their evidence regarding the physical conditions under which sedimentary formations were accumulated.