216 THE WONDERS OF GEOLOGY. Lect. III. 



adopted expressive of the characters upon which the classi- 

 fication is founded. These divisions are as follow : — 



1. The Pliocene {signifying more new or recent). — Tertiary strata, 



in which the shells are for the most part recent ; containing 

 about ten per cent, of extinct species ; these beds are subdivided 

 into the newer and older pliocene. 



2. The Miocene {denoting less recent) : — containing a small pro- 



portion, about twenty per cent., of recent species of shells. 



3. Eocene {signifying the dawn of recent, in allusion to the first 



appearance of recent species) : — containing very few recent 

 species ; not more than three or four per cent. 



The marine are in many instances associated with fresh- 

 water deposits, and the general characters of the system 

 are alternations of marine with lacustrine strata. The 

 districts occupied by these beds in Europe, are exceedingly 

 variable in extent, as Mr. Lyell has shown in a very in- 

 genious map of the tertiary seas ;* and it appears certain, 

 that during the tertiary epoch, there were large areas 

 alternately the sites of freshwater lakes and inland seas, 

 and that these transitions were dependent on oscillations in 

 the relative level of the land and water. 



13. Fossil shells. — The geological evidence afforded 

 by fossil remains has already been exemplified ; but our 

 remarks have hitherto in a great measure been restricted 

 to the relics of terrestrial quadrupeds. The shells of mol- 

 lusca, however, from their durability, often escape oblitera- 

 tion under circumstances in which all traces of the higher 

 orders of animals are lost, and they become, therefore, 

 of the utmost importance in the speculations of the geo- 

 logist. In loose sandy strata, they often occur in a high 

 degree of perfection ; in mud and clay, in a fragile state ; 

 in some instances they are silicified ; and many limestones 

 are wholly composed of shells, cemented together by calca- 



* Principles of Geology, vol. i. p. 214. 





