Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/306

Rh 292 GEOLOGY aqueous rocks, it will be of azlvantage to treat of them first, ' noting. in the first place, their original characters as result- , ing from the circumstances under which they were formed, and afterwards, the modiﬁcatioiis subsequently effected upon them. .[any of these superiiiduced structures, which are not peculiar to sedimentary. but occur more or less markedly in all rocks, may be conveniently dcscribed to- gether. The distinctive characters of the igneous or eruptive rocks, as portions of the architecture of the crust will then be described; and lastly, those of the crystalline schists and other associated rocks to which the name of metamorphic is usually applied. 1. STR.TIFICATIO.' AND ITS ACCOMP.-L'IMENTS. The term “ stratified,” so often applied as a general designation to the aqueous or sedimentary rocks, expresses their leading structural feature. They are arranged in layers or strata, an arrangement characteristic of them alike in hand-speciiiiens and in the cliffs of mountains. Not that every morsel of aqueous rock exhibits evidence of stratiﬁca- tion. But it is this characteristic which is least frequently absent. The general aspects of stratiﬁcation will be best followed in an explanation of the terms by which they are expressed. Lamince are the thinnest paper-like layers of deposit in a stratified rock. Such ﬁne layers only occur where the material is ﬁiie-graiiied, as in mud or shale, or where ﬁne scales of some mineral have been plentifully deposited, as in micaceous sandstone. In some laminated rocks the laminae cohere so ﬁrmly that they can hardly be split open, and the rock will break more readily across them than in their direction. More usually, however, the planes of lamination serve as convenient divisional planes by means of which the rock can be split open. The frequency with which lamiuee can be separated from each other, indicating, as it does, a failure of coherence between the layers of deposit, may probably be taken as a proof that these layers were originally laid down at intervals of suﬁicient duration to admit of a considerable amount of consolidation of one layer before the deposition of the next. It is quite possible that in many, if not in most cases, these intervals were of longer duration than those required for the successive deposit of the laminae. In estimating therefore the length of time represented by say one foot of such ﬁnely lami- nated rock, we might reasonably regard the actual time occupied in deposition as only a small fraction of the whole interval. The existence of laminae points to tranquil conditions of slow intermittent deposit. The sediment has been borne at intervals and fallen over the same area of undisturbed water. Regularity of thickness and persistence of litho- logical character among the laminae may be taken to iiidi— cate periodic currents, of approximately equal force, from the same quarter. In some cases successive tides in a sheltered estuary may have been the agent of deposition. In others the sediment was doubtless brought by recurring river-floods. A great thickness of laminated rock, like the massive shales of Palaeozoic formations, points to a prolonged period .of quiescence, and probably, in most cases, to slow, tranquil subsidence of the seafloor. On the other hand, the alternation of thin bands of laminated rock with others coarser in texture and non-laminated suggests consider- able oscillation of currents from different quarters bearing different qualities and amounts of sediment. Strata or Beds are layers of rock varying from an inch or less up to many feet in thickness. A stratum may be made up of many laminae, if the nature of the sediment and mode of deposit have favoured the production of this structure. This has very commonly been the case where the sediment [i v. S'l‘ltL'C'1‘l‘ R.- L. has been exceedingly fine-grained. Where the materials are of coarser grain, the strata, as a rule, are not laminated, but form the thinnest parallel divisions of the mass of rock. Strata, like laminze, may either cohere ﬁrmly, or, as more usually happens, be separable with more or less case from each other. In the former case we may suppose the upper to have followed the lower bed without the lapse of an interval long enough to allow of the consolidation of the latter. The common merging of a stratum into that which overlies it must no doubt be regarded as evidence of more or less gradual change in the conditions of deposit. Where the overlying bed shows no cohesion with that below it, the interval was probably of some duration. A stratum may be one of a series of similar beds in the same niass of rock. Thus a thick sandstone consists of niany individual strata, varying it may be very considerably in their respective thicknesses. Or a stratum may be complete and distinct in itself, as where one of limestone or ironstone run»- tlirougli the heart of a series of shales. As a general rule we may conclude that wherever among sedinientary accu- mulations stratiﬁcatioii is exceedingly well-niarked the rocks were formed rather slowly, and that where it is weak or absent the conditions of deposit were more rapid, with- out the intervals and changes necessary for the production of the distinctly stratified structure. F alse-bcc{cIz'22g, Current-l;eddz'ng.—Sonie strata, particu- larly sandstones, are marked by an irregular lamination. wherein the laniinze, though for short distances parzillel to each other, are oblique to the general stratification of the mass, at constantly varying angles and in different direc- tions. The acconipaiiyiiig section (fig. 9) illustrates this '/5‘_ T 47:‘ '**-ck, ,_'_ ~?<if"l:*%'l',’:§;i}1’fi':3.I /'* 9; _-1»  _,»,  «,4 e, '- __,.._ Fia 9.—Section of false-bedded strata on the coast of 'utci-fo;d.| structure, which is known by the name of false-bedding or current-bedding. The finer lines in this drawing represent the lamina: of deposit, the stronger lines mark successive surfaces on which these lamina: were laid down. Such a structure points to frequent changes in the direction of the currents by which the sediment was carried along and dc- posited. Sand pushed over the bottom of a sheet of water by varying currents tends to accuniulate irregularly in bands and ridges, which often advance with a steep slope in front. The upper and lower surfaces of the bank or bed of sand may remain parallel with each other as well as with the underlying bottom, yet the successive lamina: composing it may lie at an angle of 30° or even more. We may illustrate this structure by the familiar formation of a railway eni- bankment. The top of the cinbaiikment on which the permanent way is to be laid is kept level, but the advancing end of the earth-work shows a steep slope over which the 1 The woodcuts in this Part are (with the exception of Nos. 20 and 31) froin the article GEOLOGY in the last edition of the Ency. BHf., written by the late J. B. J ukes, F. R.S.