88 ERA OF THE TERTIARY FORMATION. 



pearances is one suggested by actual facts observed in the 

 strata ; that the final cretaceous beds were deposited in seas 

 more than usually deep, and which were therefore no proper 

 habitat for the animals previously existing ; that an interval 

 of time afterwards took place, which is not represented by any 

 strata which have been discovered ; x and that, by the time 

 the tertiary formation commenced, the usual modifying in- 

 fluences having never ceased, the fauna had undergone such an 

 amount of change as naturalists are accustomed to describe 

 (their language being wholly arbitrary) as a renewal of species. 



It is in perfect harmony with this view, that from the com- 

 mencement of the Tertiaries, and as we ascend in the series, 

 we find more and more specific forms identical with those still 

 existing upon earth, as if we had now reached the dawn of 

 the present state of the zoology of our planet. By the study 

 of the shells alone, Sir C. Lyell has formed a division of the 

 whole term into four sub-periods, to which he has given names 

 with reference to the proportions which they respectively pre- 

 sent of surviving species first, eocene ; second, miocene ; 

 third, older pliocene ; fourth, newer pliocene. 2 This division, 

 however, is to be regarded as not safely applicable to the Ter- 

 tiaries generally, except as a convenient means of indicating 

 various portions of the series. 



The eocene period presents, in three continental groups, 

 1238 species of shells, of which forty-two, or 3-5 per cent., 

 yet flourish unchanged. Some of these are remarkable enough ; 

 but they all sink into insignificance beside the mammalian 

 remains, which the lower eocene deposits of the Paris basin 

 present to us, showing that the land had now become the 



1 This surmise has been confirmed. " It would now appear that be- 

 tween the nummulitic tertiaries and true cretaceous strata, deposits inter- 

 vene, whose fauna and flora are such that we must regard them as of 

 tertiary age. A most interesting and important feature of these deposits, 

 traceable in the north-west of Europe, the south of France, in Savoy, in 

 Switzerland, along the southern slopes of the Alps, in Istria, and even 

 in India, is, that in numerous localities they exhibit traces of a terrestrial 

 origin, marked by the presence of coal, often accompanied by lacustrinef 

 shells, and sometimes by fresh -water limestones. In facts of this kind 

 we may get at the true explanation of the break between the cretaceous 



and tertiary faunas " Professor E. Forbes, add. to Geol. Soc., 



1854. 



He adds "I am one of those who hold, a priori, that all gaps are 

 local, and that there is a probability at some future time of our discover- 

 ing gradually somewhere on the earth's crust evidences of the missing 

 links." 



2 Lyell's Elements of Geology. 



