Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/336

Rh 3 .) .) 4-1;: This is a use of fossils, however, where great caution must be used. 'e cannot affirm that, because a certain ' species of a genus lives now in a warm part of the globe, I every species of that genus must always have lived in snnilar circumstances. The well-known example of the m-.nnmoth and woolly rhinoceros having lived in the cold north, while their modern representatives inhabit some of the warmest regions of the globe, may be usefully remembered as a warn- ing against any such conclusions. When, however, ve ﬁnd that not one fossil merely, but the whole assemblage of fossils in a formation has its modern analogue in a certain general condition of climate, we may at least tentatively infer that the same kind of climate prevailed where that assemblage of fossils lived. Such an inference would become more aml more unsafe in proportion to the antiquity of the fossils and their divergence from existing forms. 2. When the order of superposition has been determined in a great series of stratiﬁed formations, it is found that the fossils at the bottom are not quite the same as those at the top of the series. As we trace the beds upward we discover that species after species of the lowest platforms disappears, until perhaps not one of them is found. With the cessation of these older species others make their entrance. These in turn are found to die out and to be replaced by newer forms. After patient examination of the rocks, it is as- certained that every well-marked formation is characterized by its own species or genera, or by a general assemblage or facies of organic forms. This can only, of course, be deter- mined by actual practical experience over an area of some size. Vhen the typical fossils of a formation are known, they serve to identify that formation in its progress across a country. Tlms, as we trace the formation into tracts where it would be impossible to determine the true order of superposition, owing to the want of sections, or to the disturbed condition of the rocks, we can employ the fossils as a means of identiﬁcation, and speak with conﬁdence as to the succession of the rocks. We may even demonstrate that in some mountainous ground the beds have been turned completely upside down, if we can show that the fossils in what are now the uppermost strata ought properly to lie underneath those in the beds below them. Observations made over a large part of the surface of the globe have enabled geologists to divide the stratified part of the earth’s crust into systems, formations, and groups or series. These subdivisions are frequently marked off from each other by lithological characters. But mere lithological differences would afford at the best but a limited and local ground of separation. Two masses of sandstone, for ex- ample, having exactly the same general external and internal characters, might belong to very different geological periods. On the other hand, a series of limestones in one locality might be the exact chronological equivalent of a set of sand- stones and conglomerates at another, and of a series of shales and clays at a third. It is by their characteristic fossils that the divisions of the stratified rocks can be most satisfactorily made. Each formation being distinguished by its own assemblage of organic remains, it can be followed and recognized even amid the crumplings and dislocations of a disturbed region. The same general succession of organic types can be observed over a large part of the world, though, of course, with im- portant modiﬁcations in different countries. This similarity of succession has been termed Iromotaxz's—a term which ex- presses the fact that the order in which the leading types of organized existence have appeared upon the earth has been similar even in widely separated regions. It is evident that in this way a method of comparison is furnished whereby the stratified formations of different parts of the earth’s crust can be brought into relation with each GEOLOGY other. We find, for example, that a certain series of strata [’. l‘-L.-I-:0N’I'OL0(:ICAL. is characterized in Britain by certain genera aml species of corals, brachiopods, lamellibranchs, gasteropods, and cepha- lopods. A group of rocks in Bohemia, differing more or less from these in lithological aspect, contains on the whole the s:nne genera, and many even of the same species. In Scandinavia a set of beds may be seen unlike, perhaps, in external characters to the British type, but yielding many of the same fossils. In Canada and many parts of the northern United States, other rocks enclose many of the same, and of closely allied genera and species. All these groups of strata are classed together as /umzolmriul, that is, as having been deposited during the same relative period in the general progress of life in each region. It was at one time believed, aml the belief is still far from extinct, that groups of strata characterized by this com- munity or resemblance of organic remains were chrono'lo_-_ri- cally contemporaneous. But such an inference rests upon most insecure grounds. We may not be able to disprove the assertion that the strata were strictly coev-.11, but we have only to reflect on the present conditions of zoological and botanical distribution, and of 111odern sedimentation, to be assured that the assertion of contemporaneity is a mere assumption. Consider for a moment what would happen were the present surface of any portion of celitral or southern Europe to be submerged beneath the sea, covered by marine deposits, and then re—elevated into land. The river-terraces and lacustrine marls formed before the time of Julius Ctesar could not be (listinguislied by any fossil tests from those laid down in the days of Victoria, unless, indeed, traces of human implements were obtainable whereby the progress of civilization during 2000 years might be indicated. So far as regards the Sll':ll.'<,l)m1CS, and plants preserved in the various formations, it would be absolutely impossible to discriminate their relative dates: they would be classed as “geologically coxrtemporaneous,” that is, as having been formed during the same period in the history of life in the European area; yet there might be a difference of 2000 years or more between many of them. Strict contemporaneity cannot be asserted of any strata merely on the ground of similarity or identity in fossils. But the phrase “geologically contemporaneous” is too vague to have any chronological value except in a 1‘elutiV0 sense. To speak of two formations as in any sense contem- poraneous which may have been separated by thousands of years seems rather a misuse of language, though the phraseology has now gained such a footing in geological literature as probably to be inexpugnable. If we turn again for suggestions to the existing distribution of life on the earth we learn that similarity or identity of species and genera holds good on the whole only for limited areas, and consequently, if applied to wide geographical regions, ought to be an argument for diversity rather than for simil-.n' ty of age. If we suppose the British seas to be raised int» dry land, so that the organic relics preserved in their sands and silts could be exhumed and examined, a general cu »m- mon facies or type would be found, though some spot-its would be more a.bundant in or entirely confined to the north, while others would show a greater development in the opposite quarter. Still there would be such a siniil-11-i.y throughout the whole that no naturalist would hesitate to regard the organisms as those of one biological province, and belonging to the same great geological period. The region is so small, and its conditions of life so uniform and uninterrupted, that no marked distinction is possible between the forms of life in its different parts. Widening the area of observation, we perceive that as we recede from any given point the forms of life gradually change. Vegetation alters its aspect from climate to climate, and with it come corresponding transformations in the character of insects, birds, and wild animals. A lake bottom