THE EOCENE PERIOD. 295 



spicuous features in the Cretaceous fauna, render the palseon- 

 tological break between the Chalk and the Eocene one far too 

 serious to be overlooked. At the same time, it is to be re- 

 membered that the evidence afforded by the explorations car- 

 ried out of late years as to the animal life of the deep sea, ren- 

 ders it certain that the extinction of marine forms of life at the 

 close of the Cretaceous period was far less extensive than had 

 been previously assumed. It is tolerably certain, in fact, that 

 we may look upon some of the inhabitants of the depths of our 

 existing oceans as the direct, if modified, descendants of ani-* 

 mals which were in existence when the Chalk was deposited. 



It follows from the general want of conformity between the 

 Cretaceous and Tertiary rocks, and still more from the great 

 difference in life, that the Cretaceous and Tertiary periods are 

 separated, in the Old World at any rate, by an enormous lapse 

 of unrepresented time. How long this interval may have been, 

 we have no means of judging exactly, but it very possibly was 

 as long as the whole Kainozoic epoch itself. Some day we 

 shall doubtless find, at some part of the earth's surface, marine 

 strata which were deposited during this period, and which will 

 contain fossils intermediate in character between the organic 

 remains which respectively characterize the Secondary and 

 Tertiary periods. At present, we have only slight traces of 

 such deposits as, for instance, the Maestricht beds, the Faxoe 

 Limestone, and the Pisolitic Limestone of France. 



CLASSIFICATION OF THE TERTIARY ROCKS. The classifica- 

 tion of the Tertiary rocks is a matter of unusual difficulty, in 

 consequence of their occurring in disconnected basins, form- 

 ing a series of detached areas, which hold no relations of 

 superposition to one another. The order, therefore, of the 

 Tertiaries in point of time, can only be determined by an ap- 

 peal to fossils; and in such determination Sir Charles Lyell 

 proposed to take as the basis of classification the proportion of 

 living or existing species of Mollusca which occurs in each stra- 

 tum or group of strata. Acting upon this principle, Sir Charles 

 Lyell divides the Tertiary series into four groups : 



I. The Eocene formation (Gr. eos, dawn; kainos, new), con- 

 taining the smallest proportion of existing species, and being, 

 therefore, the oldest division. In this classification, only the 

 Mollusca are taken into account; and it was found that of 

 these about three and a half per cent were identical with ex- 

 isting species. 



