286 HISTORICAL PAL/EONTOLOGY. 



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 characterise 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 propoHion of 

 living or existing species of Mo II us ca ivhich 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 

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

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

 isting species. 



XL The Mioceiie formation (Gr. meion, less ; kainos, new), 

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

 ceeding formation, and less than one-half the total number in the 

 formation. As before, only the MoUusca are taken into account, 

 and about 17 per cent of these agree with existing species. 



III. The Pliocene formation (Gr. pleion, more ; kai?ios, new), 

 with generally 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 Formatio?is, in which all the shells 

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

 minor groups — the Post-Pliocene and Receiit For??tations. In 

 the Post-Pliocene formations, while all the MoUusca belong to 

 existing species, most of the Mam?nals belong to extinct 

 species. In the Recent period, the quadrupeds, as well as the 

 shells, belong to living species. 



The above, with some modifications, was the original classi- 

 fication proposed by Sir Charles Lyell for the Tertiary rocks, 

 and now universally accepted. More recent researches, it is 

 true, have somewhat altered the proportions of existing species 



