Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/340

Rh 5. The relative chronological value of the divisions of the geological record is not to be measured by mere depth of strata. While it may be reasonably assumed that a great thickness of stratiﬁed rock nmst mark the passage of a long period of time, it cannot safely be aﬁirmed thata much less thickness elsewhere represents a correspondingly diminished period. This may sometimes be made evident by an uncon- formability between two sets of rocks, as has already been explained. The total depth of both groups together may be, say 1000 feet. Elsewhere we may ﬁnd a single un- broken formation reaching a depth of 10,000 feet ; but it would be utterly erroneous to conclude that the latter repre- sents ten times the length of. time shown by the two former. So far from this being the case, it might not be diﬁicult to show that the minor thickness of rock really denoted by far the longer geological interval. If, for instance, it could be proved that the upper part of both the sections lay on one and the same geological platform, but that the lower unconformable series in the one locality belonged to a far lower and older system of rocks than the base of the thick conformable series in the other, then it would be clear that the gap marked by the nnconformability really indicate(l a longer period than the massive successionof deposits. 6. Fossil evidence furnishes the chief means of comparing the relative value of formations and groups of rock. A break in the succession of organic remains marks an inter- val of time often unrepresented by strata at the place where the break is found. The relative importance of these breaks, and therefore, probably, the comparative intervals of time which they mark, may be estimated by the difference of the facics of the fossils on each side. If, for example, in one case we ﬁnd every species to be dissimilar above and below a certain horizon, while in another locality only half of the species on each side are peculiar, we naturally infer, if the total number of species seems large enough to warrant the inference, that the interval marked by the former break was very much longer than that marked by the second. But we may go further and compare by means of fossil evidence the relation between breaks in the succession of organic remains anrl the depth of strata between them. Three formations of fossiliferous strata, A, C, and H, may occur conformably above each other. By a comparison of the fossil contents of all parts of A, it may be ascertained that, while some species are peculiar to its lower, others to its higher portions, yet the majority extend throughout the formation. If new it is found that of the total num- ber of species in the upper portion of A only one-third passes up into C, it may be inferred with some probability that the time represented by the break between A and C wis really longer than that required for the accumulation of the whole of the formation A. It might even be pos- sible to discover elsewhere :3. thick intermediate formation 1} filling up the gap between A and C‘-. In like manner were it to be discovered that, while the whole of the forma- tion C is characterized by a common suite of fossils, not one of the species and only one half of the genera pass up into B, the inference could hardly be resisted that the gap between the two formations marks the passage of a far longer interval than was needed for the deposition of the whole of C. And thus we reach the remarkable con- clusion that, thick though the stratiﬁed formations of a country may be, in some cases they may not represent so long a total period of time as do the gaps in their suc- cession,—iu other words, that non-deposition was more frequent and prolonged than deposition, or that the intervals of time which have been recorded by strata have not been so long as those which have not been so recorded. In all speculations of this nature, however, it is necessary to reason from as wide a basis of observation as possible, seeing that so much of the evidence is negative. Especially GEOLOGY [vr. STRATIGR.-XPIIICAL. needful is it to bear in mind that the cessation of one or more species at a certain line among the rocks of a particu- lar district may mean nothing more than that, owing to some change in the conditions of life or of deposition, these species were compelled to migrate or became locally ex- tinct at the time marked by that line. They may have con- tinued to flourish abundantly in neighbouring districts for a long period afterward. ZI-any examples of this obvious truth might be cited. Thus in a great succession of mingled marine, brackish-water, and terrestrial strata, like that of the Carboniferous Limestone series of Scotland, corals, crinoids, and brachiopods abound in the limestoncs and accompanying shales, but disappear as the sandstones, ironstones, clays, coals, and bituminous sh-ales snpervcne. An observer meeting for the ﬁrst time with an instance of the disappearance, and remembering what he had read about “breaks in succession,” might be tempted to specu- late about the extinction of these organisms, and their re- placement by other and later forms of life, such as the ferns, lycopods, ganoid ﬁshes, and other fossils so abundant in the overlying strata. But further research would show him that high above the plant—bearing sandstones and coals other limestones and shales might be observed, once more charged with the same marine fossils as before, and still farther overlying groups of sandstones, coals, and carbona- ceous beds followed by yet higher marine limestones. He would thus learn that the same organisms, after being locally extcrminated, returned again and again to the same area. After such a lesson he would probably pause before too conﬁdently asserting that the highest bed in which we can detect certain fossils marked really their ﬁnal appearance in the history of life. A break in the succession may thus be extremely local, one set of organisms having been driven to a different part of the same region, while another set occupied their place until the ﬁrst was enabled to return. 7. The geological record is at the best but an imperfect chronicle of tliegerilogical history of the earth. It abounds in gaps, some of which have been caused bythe destruction of strata owing to metamorphism, denudation, or Olll('I‘'lS(‘, some by original non-deposition, as above explained. Nevertheless from this record alone can the progress of the earth. be traced. It contains the registers of the births and deaths of tribes of plants and animals which have from time to time lived on the earth. But a small proportion of the total number of species which have appeared in past time have been thus chronicled, yet by collecting the broken fragments of the record an outline at least of the history of life upon the earth can be dccipbered. The nomenclature adopted for the subdivisions of the geological record bears witness to the rapid growth of geo- logy. It is a patch—work in which no system nor language has been adhered to, but where the inﬂuences by which the progress of the science has been moulded may be distinetly traced. Some of the earliest names are lithologi- cal, and remind us of the fact that mineralogy and petro- graphy preceded geology in the order of birth—- (‘lralk, Oolite, Greensand, Millstone Grit. Others are topog1-a- phical, and often recall the labours of the early geologists of l§nrrland—London Clay, Oxford C ‘lay, Purbcck, Portland, Kimeridge beds. Others are taken from local linglish provincial names, and remind us of the debt we owe to Villiam Smith, by whom so many of them were ﬁrst used —Lias, Gault, (‘rag, Cornbrash. Others of later date recog- nize an order of superposition as already established among formations—Old lied Sandstone, New lied Sandstone. By common consent it is admitted that names taken from the region where a formation or group of rocks is typically developed, are best adapted for general use. Calnbrian, Silurian, Devonian, Permian, Jurassic, are of this class, and have been adopted all over the globe.