Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/309

Rh srRA'rIFIcA'rioN.] so concretionary as to resemble masses of conglomerate; yet the coiicretions, among all their fantastic shapes and with their acquired crystalline texture, may often be found to retain traces of the original stratiﬁcatioii of the rock. Beds of rock-salt may likewise be observed to be marked with traces of a concretioiiary arrangement. Order of ;S'uperposL'tz'o7e——t/re Fozmclation of Geological ('lzronoIo_r/_7/.—As sedimentary strata are laid down upon one another in a more or less nearly horizontal position, the underlying beds must be older than those which cover them. This simple and obvious truth is termed the law of super- position. It furnishes the means of determining the chronology of rocks, and though other methods of as- certaining this point are employed, they must all be based originally upon the observed order of superposition. The only case where the apparent superposition may be decep- tive is where the strata have been inverted. In the Alps, for example, the rocks composing huge mountain masses have been so completely overturned that the highest beds ﬂppC'1l‘ as if regularly covered by others which ought pro- perly to underlie them. But these are exceptional occur- rences, where the true order can usually be made out from other sources of evidence. Allernat-ions of Stratr(.—Though great variations occur in the nature of the strata composing a mass of sediment- ary rocks, it may often be observed that certain repe- titions occur. Saiidstones, for example, are found to be interleaved with shale above, and then to pass in to shale; the latter may in turn become sandy at the top and be ﬁnally covered by sandstone, or may assume a calcareous character and pass up into limestone. Such alternations bring before us the conditions under which the sedimentation took place. A sandstone group indicates water of comparatively little depth, moved by changing currents, bringing the sand now from one side now from another. The passage of such a group into one of shale points to a diminution in the motion and transporting power of the water, perhaps to a sinking of the tract, whereby only ﬁne mud was then intermittently brought into it. The advent of a limestone above the shale serves to show that the water cleared, owing to a deflexion of the sediment—carrying currents, or to continued and perhaps more rapid subsidence, and that Formnz'm'fera, corals, crinoids, Jlullusrn, or other lime-secreting organisms, established themselves upon the spot. Shale overlying the limestone would tell of fresh inroads of mud, which destroyed the animal life that had been ﬂourishing on the bottom; while a return of sandstone beds would mark how, in the course of time, the original conditions of troubled currents and shifting sandbaiiks returned. Such alterna- ting groups of sandy, calcareous, and argillaceous strata are well illustrated among the Jurassic formations of England. Associa(z'ons Qf St;-ata.—Certain kinds of strata very coni- monly occur together, because the conditions under which they were formed were apt to arise in succession. One of the most familiar examples is the association of coal and ﬁre-clay. A seam of coal is almost invariably found to lie on a bed of ﬁre-clay, or on some argillaceous stratum. The reason of this union becomes at once apparent when we learn that the ﬁre-clay formed the soil on which the plants grew that went to form the coal. Where the clay was laid down under suitable circumstances vegetation sprang up upon it. Again conglomerate and sandstone occur together rather than conglomerate and shale, because the agitation of the water which could form and deposit coarse detritus, like that composing conglomerate, was too great to admit of the accumulation of ﬁne silt. For a similar reason we may look for shale or clay rather than sandstone as an accompaniment of limestone. Ii’cZrm've [’cI‘sistc72ce of Sh'ata.—Observation of what takes place on any lake bottom, estuary, or sea-margin GEOLOGY 295 teaches that some kinds of sediment are much more widely spread than others, and prepares us to ﬁnd that the same has been the case in past time, and therefore that some kinds of sedimentary rocks possess far greater persistence than others. As a general rule it may be said that the coarser the grain the more local the extent of a rock. Conglomerates are thus by much the most variable and inconstaiit of all sedimentary formations. They suddenly sink down from a thickness of several hundred feet to a few yards, or die out altogether, to reappear perhaps further on, in the same wcdge—like or len- ticular fashion. Sandstoncs are less liable to such extremes of inconstancy, but they too are apt to thin away and to swell out again. Shales are much more persistent, the same zone being often traceable for many miles. Liinestoncs sometimes occur in thick local masses, as among the Silurian formations of Wales and Scotland, but they often also display remarkable continuity. Three thin limestone bands, each of them only 2 or 3 feet in thickness, and separated by a considerable thickness of intervening sand- stones aiid shales, can be traced through the coal-ﬁelds of central Scotland over an area of at least 1000 square miles. Coal—seams also possess great persistence. The same seams, varying slightly in thickness and quality, may often be traced thoughout the whole of an extensive coal-ﬁeld. What is thus true of individual strata may be aﬂirmed also of groups of such strata. A thick mass of sandstone will be found as a. rule to be more continuous than one of con- glomerate, biit less so than one of shale. A series of lime- stone-beds will usually be found to stretch further than either of them. But even to the most. extensive stratum or group of strata there must be a limit. It must end off and give place to others, either suddenly, as abank of shingle is succeeded by the sheet of sand heaped against its base, or very gradu- ally, by insensibly passing into other strata on all sides. Great variations in the character of stratiﬁed rocks may frequently be observed in passing from one part of a country to another along the outcrop of the same rocks. Thus at one end we may meet with a thick series of sand- stones and shales which, traced in a certain direction, may be found passing into limestones. A group of strata may consist of massive conglomerates at one locality, and may graduate into ﬁne ﬁssile ﬂagstones in another. A thick mass of clay may be found to alternate more and more with shelly sands as it is traced outward, until it loses its argillaceous nature altogether. No diﬁiculty need be felt in admitting the strict contemporaneity of these diverse layers of sediment. At the present time we see how coarse shingle may be formed along the beach at the same time that the ﬁnest mud is being laid down on the same sea- bottom further from land. Could we raise up that bottom, we should doubtless find as gradual a. passage from the littoral to the deeper water deposits as we do among the geological formations of the earth’s crust. The existing diﬁ'erences of character between the deposits of the shore and of the opener sea would no doubt continue to be main- tained, with slight geographical displacements, even if the whole area were undergoing subsidence, giving rise to a thick group of littoral beds in one tract and of deeper—water ac- cumulations at another. In like manner among the forma- tions of former geological periods the same conditions of de- posit appear soinetimes to have continued for a considerablc period. Hence the thick Mountain or Carboniferous Lime- stone of Derbysliire is gradually replaced northwards by the thick sandstoneshales, ironstones, and coal-seams of Scotland. Ozverlap.-Wheii strata have been laid down in a subsid- ing region wherein the area of deposit ‘gradually increased, the sediment must have spread over a progressively augment- ing surface. By this means the later portions of a. sedi- mentary series will extend beyond the limits of the older