« ПредыдущаяПродолжить »
nesses of the systems of strata, but yet (as will be shewn hereafter) undoubtedly to some extent represented by them.
In this scale the total thickness of the Fossiliferous strata, down to the Lingula beds of Wales, is assumed to be 10 miles, 52800 ft. That is not the maximum thickness, which in Britain is supposed by Ramsay to be 72584 ft. In his estimate the Palæozoic beds are taken much above the average, and the Cenozoic beds much below the average of Europe. Morris gives the maximum numbers thus, Cenozoic 2830, Mesozoic 6170, Palæozoic 49460. D'Orbigny's general statement gives 21260 metres =69750 ft., the Tertiary occupying 9842 ft., while in England they are supposed to be 2240. The figures on the left may be used to mark miles of depth, thickness of deposits, or periods of unknown duration, according to the purpose in view.
The Geological Scale of Time thus constituted by the succession of marine strata, is liable to the objection which applies to almost every scale of historical time, that it is not complete in any one region, no one oceanic basin having been yet discovered which has received marine sediments continuously through all geological periods. The remedy is the same as in ordinary history-the scales of different regions are combined by means of common terms, which in one
district appear in the upper part of one series, and in another district in the lower part of another. Thus in several parts of England the Permian system is unconformed to the Coal series, but on the borders of the Staffordshire Coal-field it seems otherwise, and the sequence appears to be nearly complete. Again, the passage of the Permian strata into the Triassic group is more full in the south of Yorkshire than elsewhere, and the passage of the Trias into Lias is best exhibited (in England) in the Vale of the Severn. The uppermost chalk of Maestricht and the region of Bayonne softens the break which in England appears so decisive at the base of the Canozoic strata; and thus, with few exceptions, by taking our data from different regions we acquire nearly but not quite a complete series of the strata of marine origin, and the means of accurately placing the monuments of the life which has passed away for ever.
SUCCESSIVE SYSTEMS OF LIFE.
Passing from the limited consideration of man and his works, and the animals and plants specially associated with him, to examine as a third problem how far into antiquity we can trace the forms of the numerous species of existing plants and animals, we find in deposits of old, but perhaps not prehistoric
or at least prehuman date-peat bogs-subterranean forests-sea-beaches-lacustrine marls-river-sediments-many examples of the plants, shells, insects, birds and quadrupeds now existing, together with a few no longer living in the same regions, as the Wolf and Beaver in England, or not now living any where, as the Irish Elk, Rhinoceros, Hippopotamus major, Elephas primigenius, and large Lion. Below these, in the deposits called Pleistocene, the proportion of extinct species mixed with species still living increases. In the Pleiocene tertiaries it is great, still greater in the Meiocene, and quite predominant in the Eocene, below which, in all the Mesozoic and Paleozoic Strata, there is hardly a fossil specimen of a living species-though it is held by several writers that Terebratulina striata of the Cretaceous Strata is both fossil and living, and it is not possible to distinguish some forms of fossil and living Foraminifera1.
With the exceptions already noted, if they be really such, the whole living creation, examined by species, as we understand this term, ceases below the Tertiary Strata. For this reason, to indicate the real affinity of their organic contents to the living creation, I have elsewhere called them Cenozoic.
The following table will illustrate this statement 1 Auct. Ehrenberg, Jones, Parker.
by a few examples of living species selected from marine animals only1.
But if we examine the fossil and living races by genera instead of species, the result is different. For instance, some existing genera of Mollusca occur in the Mesozoic, and perhaps even in the Paleozoic Strata, though in these latter the genera Avicula, Modiola, Nucula, &c., among Conchifera, are by some writers thought to be distinguishable from their living analogues.
The subjoined table is meant to illustrate this
1 The asterisk indicates in what deposits the living species occur. E. Echinodermata; A. Annelida; Cr. Crustacea; P. Polyzoa or Bryozoa; B. Brachiopoda; M. Monomyaria; D. Dimyaria; G. Gasteropoda; Cet. Cetacea.
statement by examples taken from the same classes as in the former table, with the exception of Fishes (F), which are here substituted for Cetacea.
For comparison with the tables just given of species and genera still living traced backward in time, we may in like manner select species and genera living in the older strata, and trace them upward in the rocks, or forward in time, as in the following examples.
Fishes in the