EOCENE PERIOD. 547 



the modern classification of the Kainozoic rocks, propounded 

 by Sir Charles Lyell. 



It follows from the constant want of conformity between 

 the Cretaceous and Tertiary rocks, and still more from the 

 entire difference in life, that the Cretaceous and Tertiary 

 periods are separated 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 doubt- 

 less find, at some part of the earth's surface, strata which were 

 deposited during this period, and which will contain fossils 

 intermediate in character between the organic remains which 

 respectively characterise the Secondary and Tertiary periods. 

 At present, we have only slight traces of such deposits, as, for 

 instance, the Maestricht beds. 



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 

 appeal 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 stratum 

 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 

 existing species. 



II. The Miocene formation (Gr. meion, less ; kainos, new), 

 with more recent species than the Eocene, but less than the 

 succeeding formation, and less than one-half the total number 

 in the formation. As before, only the Mollusca are taken into 

 account, and about 17 per cent of these agree with existing 

 species. 



III. The Pliocene formation (Gr. pleion, more ; kainos, new), 

 with more than half the species of shells identical with existing 

 species ; the proportion of these varying from 35 to 50 per 

 cent in the lower beds of this division, up to 90 or 95 per cent 

 in its higher portion. 



IV. The Post-Tertiary Formations, in which all the shells 

 belong to existing species. This, in turn, is divided into two 



