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PART VII
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See also:PART VII .—STRATIGRAPmCAL See also:GEOLOGY
This See also:branch of the See also:science arranges the rocks of the See also:earth's crust in the See also:order of their See also:appearance, and interprets the sequence of events of which they See also:form the records. Its See also:province is to cull from the other departments of geology the facts which may be needed to show what has been the progress of our See also:planet, and of each See also:continent and See also:country, from the earliest times of which the rocks have preserved any memorial. Thus from See also:mineralogy and petrography it contains See also:information regarding the origin and subsequent mutations of minerals and rocks. From dynamical geology it learns by what agencies the materials of the earth's crust have been formed, altered, broken, upheaved and melted. From geotectonic geology it understands the various processes whereby these materials were put together so as to build up the complicated crust of the earth. From palaeontological geology it receives in well-determined fossil remains a See also:clue by which to discriminate the different stratified formations, and to trace the See also:grand onward See also: A See also:bed, or a number of beds, linked together by containing one or more distinctive See also:species or genera of fossils is termed a See also:zone or See also:horizon, and usually bears the name of one of its more characteristic fossils, as the Planorbis-zone of the See also:Lower See also:Lias, which is so called from the prevalence in it of the ammonite Psiloceras planorbis. Two or more such zones related to each other by the See also:possession of a number of the same characteristic species or genera have been designated beds or an See also:assise. Two or more sets of beds or assises similarly related form a See also:group or See also:stage; a number of groups or stages make a series, formation or See also:section, and a See also:succession of formations may be See also:united into a See also:system. 3. Some living species of See also:plants and animals can be traced downwards through the more See also:recent geological formations; but the number which can be so followed grows smaller as the examination is pursued into more See also:ancient deposits. With their disappearance other species or genera See also:present themselves which are no longer living. These in turn may be traced backward into earlier formations, till they too cease and their places are taken by yet older forms. It is thus shown that the stratified rocks contain the records of a See also:gradual progression of organic forms. A species which has once died out does not seem ever to have reappeared. 4. When the order of succession of organic remains among the stratified rocks has been determined, they become an invaluable See also:guide in the investigation of the relative See also:age of rocks and the structure of the See also:land. Each zone and formation, being characterized by its own species or genera, may be recognized by their means, and the true succession of strata may thus be confidently established even in a country wherein the rocks have been shattered by dislocation, folded, inverted or metamorphosed. 3. Though See also:local See also:differences exist in regard to the precise zone in which a given species of organism may make its first appearance, the See also:general order of succession of the organic forms found in the rocks is never inverted. The record is nowhere See also:complete in any region, but the portions represented, even though extremely imperfect, always follow each other in their proper See also:chronological order, unless where disturbance of the crust has intervened to destroy the See also:original sequence. 6. The relative chronological value of the divisions of thegeological record is not to be measured by See also:mere See also:depth of strata. While it may be reasonably assumed that, in general, a great thickness of stratified See also:rock must See also:mark the passage of a See also:long See also:period of See also:time, it cannot safely be affirmed that a much less thickness elsewhere must represent a correspondingly diminished period. The need for this caution may sometimes be made evident by an unconformability between two sets of rocks, as has already been explained. The See also:total depth of both groups together may be, say r000 ft. Elsewhere we may find a single unbroken formation reaching a depth of ro,000 ft.; but it would be unwarrantable to assume that the latter represents ten times the length of time indicated by the former two. So far from this being the See also:case, it might not be difficult to show that the See also:minor thickness of rock really denotes by far the longer geological . See also:interval. If, for instance, it could be proved that the upper part of both the sections lies on one and the same geological See also:platform, but that the lower unconformable series in the one locality belongs to a far lower and older system of rocks than the See also:base of the thick conformable series in the other, then it would be clear that the See also:gap marked by the unconformability really indicates a longer period than the massive succession of deposits. 7. Fossil See also:evidence furnishes the See also:chief means of comparing the relative value of formations and groups of rock. A " break in the succession of organic remains," as already explained, marks an interval of time often unrepresented by strata at the place where the break is found. The relative importance of these breaks, and therefore, probably, the See also:comparative intervals of time which they mark, may be estimated by the difference of the facies or general character of the fossils on each See also: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 See also:half of the species on each side are See also:peculiar, we naturally infer, if the total number of species seems large enough to See also:warrant the inference, that the interval marked by the former break was 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 and 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 See also:majority extend throughout the formation. If now it is found that of the total number of species in the upper portion of A only one-third passes up into C, it may be inferred with some plausibility that the time represented by the break between A and C was really longer than that required for the See also:accumulation of the whole of the formation A. It might even be possible to discover elsewhere a thick inter-mediate formation B filling up the gap between A and C. In like manner were it to be discovered that, while the whole of the formation C is characterized by a See also:common See also:suite of fossils, not one of the species and only one half of the genera pass up into H, 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 conclusion 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 succession,—in 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 See also:reason from as wide a basis of observation as possible, seeing that so much of the evidence is negative. Especially needful is it to See also:bear in mind that the cessation of one or more species at a certain See also:line among the rocks of a particular See also:district may mean nothing more than that, onward from the time marked by that line, these species, owing to some See also:change in the conditions of See also:life, were compelled to migrate or became locally See also:extinct or, from some alteration in the conditions of fossilization, were no longer imbedded and preserved as fossils. They may have continued to flourish abundantly in neighbouring districts for a long period afterward. Many examples of this obvious truth might be cited. Thus in a great succession of mingled marine, brackish-See also:water and terrestrial strata, like that of the Carboniferous See also:Limestone series of See also:Scotland, See also:corals, crinoids and brachiopods abound in the limestones and accompanying shales, but disappear as the sandstones, ironstones, See also:clays, coals and bituminous shales supervene. An observer See also:meeting for the first time with an instance of this disappearance, and remembering what he had read about breaks in succession, might be tempted to speculate about the extinction of these organisms, and their replacement by other and later forms of life, such as the ferns, lycopods, estuarine or fresh-water shells, 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 carbonaceous beds followed by yet higher marine limestones. He would thus learn that the same organisms, after being locally exterminated, returned again and again to the same See also:area. After such a See also:lesson he would probably pause before too confidently asserting that the highest bed in which we can detect certain fossils marks their final appearance in the history of life. Some breaks 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. 8. The geological record is at the best but an imperfect See also:chronicle of the geological history of the earth. It abounds in gaps, some of which have been caused by the destruction of strata owing to See also:metamorphism, denudation or otherwise, others 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 appearance and disappearance of tribes of plants and animals which have from time to time flourished on the earth. Only a small proportion of the total number of species which have lived in past time have been thus chronicled, yet by See also:collecting the broken fragments of the record an outline at least of the history of life upon the earth can be deciphered. It cannot be too frequently stated, nor too prominently kept in view, that, although gaps occur in the succession of organic remains as recorded in the rocks, they do not warrant the conclusion that any such See also:blank intervals ever interrupted the progress of plant and See also:animal life upon the globe. There is every reason to believe that the march of life has been unbroken, onward and upward. Geological history, therefore, if its records in the stratified formations were perfect, ought to show a blending and gradation of See also:epoch with epoch. But the progress has been constantly interrupted, now by upheaval, now by volcanic outbursts, now by depression. These interruptions serve as natural divisions in the chronicle, and enable the geologist to arrange his history into periods. As the order of succession among stratified rocks was first made out in See also:Europe, and as many of the gaps in that succession were found to be widespread over the See also:European area, the divisions which experience established for that portion of the globe came to be regarded as typical, and the names adopted for them were applied to the rocks of other and far distant regions. This application has brought out the fact that some of the most marked breaks in the European series do not exist elsewhere, and, on the other See also:hand, that some portions of that series are much more complete than the corresponding sections in other regions. Hence, while the general similarity of succession may remain, different subdivisions and nomenclature are required as we pass from continent to continent. The nomenclature adopted for the subdivisions of the geological record bears See also:witness to the rapid growth of geology. It is a patch-See also:work in which no system nor See also:language has been adhered to, but where the influences by which the progress of the science has been moulded may be distinctly traced. Some of the earliest names are lithological, and remind us of the fact that mineralogy and petrography preceded geology in the order of See also:birth—See also:Chalk, Oolite, See also:Greensand, Millstone Grit. Others are topographical, and often recall the labours of the See also:early geologists of See also:England—See also:London See also:Clay, See also:Oxford Clay, Purbeck, See also:Portland, Kimmeridge beds. Others are taken from local See also:English provincial names, andremind us of the See also:debt we owe to See also: The stratified formations of the earth's crust, or geological record, are classified into five See also:main divisions, which in their order of antiquity are as follows: (I) Archean or Pre-Cambrian, called also sometimes Azoic (lifeless) or Eozoic (See also:dawn of life); (2) Palaeozoic (ancient life) or See also:Primary; (3) Mesozoic (See also:middle life) or Secondary; (4) See also:Cainozoic (recent life) or See also:Tertiary; (5) See also:Quaternary or See also:Post-Tertiary. These divisions are further ranged into systems, formations, groups or stages, assises and zones. Accounts of the various subdivisions named are given in See also:separate articles under their own headings. In order, however, that the sequence of the formations and their See also:parallelism in Europe and See also:North See also:America may be presented together a strati-graphical table is given on next See also:page. Additional information and CommentsThere are no comments yet for this article.
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