Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/310

Rh 296 parts, and will repose directly upon the shelving bottom, with none of those older strata underneath them. This relation is called Overlap (see ﬁg. 59). The higher or newer members are said to overlap the older. This structure may often be detected among formations of all geological ages. It brings before us the shore line of ancient land-surfaces, and shows how, as these sank under water, the gravels, sands, and silts gradually advanced and covered them. Relative Lapse of Time represented by Strata and by Ike Intervals between tI¢em.—Of the absolute length of time represented by any strata or groups of strata we call form no satisfactory estimates. Certain general conclu- sions may indeed be drawn, and comparisons may be made between different series of rocks. Sandstones full of false-bedding were probably accumulated more rapidly than ﬁnely—laminated shales or clays. It is not uncommon in certain Carboniferous formations to ﬁnd huge coniferous trunks imbedded in an inclined position in sandstone. These trees seem to have been carried along and to have sunk, their heavier or root-end touching the bottom, and their upper end pointing upward in the direc- tion of the current, exactly as in the case of the snags of the Mississippi. The continuous deposit of sand at last rose above the level of the trunks and buried them. It is clear then that the rate of deposit must have been sufficiently rapid to have allowed a mass of 20 or 30 feet of sand to accumulate before the decay of the wood; though modern instances are known where, under certain circumstances, submerged trees may last for some centuries. (‘ontinuous layers of the same kind of deposit suggest a persistence of geological conditions; numerous alterna- tions of different kinds of sedimentary matter point to vicissitudes or alternations of conditions. As a rule, we should infer that the time represented by a given thickness of similar strata was less than that shown by the same thickness of dissimilar strata, because the changes needed to bring new varieties of sediment into the area of deposit would usually require the lapse of some time for their completion. But this conclusion might often be erroneous. It would be best supported when, from the very nature of the rocks, wide variations in the character of the water- bottom could be established. Thus a group of shales followed by a fossiliferous limestone would almost always mark the lapse of a much longer period than an equal depth of sandy strata. Limestones made up of organic remains which lived and died upon the spot, and whose remains are crowded together generation above generation, must have demanded many years for their formation. But in all speculations of this kind we must bear in mind that the length of time represented by a given depth of strata is not to be estimated merely from their thickness or lithological characters. It has already been pointed out that the interval between the deposit of two successive laminae of shale may have been as long as, or even longer than, that required for the formation of one of the laminze. In like manner, the interval needed for the transition from one stratum or kind of strata to another may often have been more than equal to the time required for the formation of the strata on either side. But the relative chronological importance of the bars or lines in the geological record can seldom be satisfactorily discussed merely on lithological grounds._ This must mainly be decided on the evidence ot organic remains, as will be shown in part v. By this kind of evidence it can be made nearly certain that the intervals represented by strata were in many cases much shorter than those not so represented,—in other words, that the time during which no deposit of sediment went on was longer than that wherein deposit did take place. Groups of ;S'tratu.——Passing from individual strata to large masses of stratiﬁed rock, the geologist finds it needful GEOLOGY [1v. STRUCTURA L. ' for convenience of reference to subdivide these into groups. ' Ile avails himself of two bases of classiﬁcation-—(l) litho- logical characters, and (2) organic remains. 1. The subdivision of stratiﬁed rocks into groups accord- ing to their mineral aspectis an obvious a11d easily applied classiﬁcation. Moreover, it often serves to connect together rocks formed continuously in certain circumstances which differed from those under which the strata above and below were laid down,—so that it expresses natural and original subdivisions of strata. In the middle of the English Car- boniferous system of rocks, for example, a zone of sandy and pebbly beds occurs, known as the Millstone Grit. l'o abrupt and sharp line can be drawn between these strata and those above and below them. They shade upward and downward into the beds between which they lie. Yet they form a conspicuous belt, traceable for many miles by the ,scenery to which it gives rise. The red rocks of central England, with their red sandstones, marls, rock-salt, and gypsum, form likewise a well-marked group or rather series i of groups. It is obvious, however, that characters of this ' kind, though sometimes wonderfully persistent over wide tracts of country, must be at best but local. The physical , conditions of deposit must always have been limited in ex- tent. A group of strata showing great thickness in one region will be found to die away as it is traced into I another. Or its place is gradually taken by another group which, even if geologically contemporaneous, possesses totally ditferent lithological characters. Just as at the I present time a group of sandy deposits gradually gives ' place along the sea-ﬂoor to others of mud, and these to ' others of shells or of gravel, so in former geological periods contemporaneous deposits were not always lithologically similar. Hence mere resemblance in mineral aspect usually 'cannot be regarded as satisfactory evidence of conte1n- iporaneity except within comparatively contracted areas. careous group of rocks, full of corals, crinoids, a11d other organisms, which bear witness to the formation of these rocks in the open sea. But if these limestones, with their characteristic marine fossils, are traced into the north of ' England and Scotland, they are found to pass into sand- stones and shales, with numerous coal-seams, and only a few thin beds of limestone. The soft clay beneath tl1e city of London is represented in the Alps by hard schists and - contorted limestones. We conclude therefore that litho- logical agreement when pushed too far is apt to mislead us, partly because contemporaneous strata often vary greatly in their lithological character, and partly because the same lithological characters may appear again and again in dif- ferent ages. By trusting too implicitly to this kind of evidence, we may be led to class together rocks belonging to very different geological periods, and on the other hand ' to separate groups which really, in spite of their seeming distinction, were formed contemporaneously. 2. It is by the remains of plants and animals imbedded among the stratiﬁed rocks that the most satisfactory sub- , divisions of the geological record can be 1nade, as will be more fully stated in parts v. and vi. A chronological sue I cession of organic forms can be made out among the rocks of, the earth’s crust. A certain common facies or type of fossils is found to characterize particular groups of rock, and to hold true even though the lithological constitution of the strata should greatly vary. Moreover, though comparatively few species are universally diffused, they possess remarkable persistence over wide areas, and even when they are re- placed by others, the same general facies of fossils remains. ‘ Hence the stratiﬁed formations of two countries geographi-
 * The Carboniferous Limestone of Ireland is a thick cal-
 * cally distant, and having little or no lithological resemblance

i to each other, may be compared and paralleled zone by zone,
 * simply by means of their enclosed organic remains.