94 TERTIARY FORMATIONS. 



ation to formation, so that they have been said, almost without 

 exaggeration, to be as regularly disposed in the geological for- 

 mations as in the drawers of a well-arranged museum ; hence 

 if the fossils of any given locality be known, we may securely 

 pronounce as to its geological formation, and vice versa."* 



Mr. Lyell has divided the tertiary formations into three 

 periods ; and it has been ascertained by Deshayes, that as 

 we proceed from the lowest of these to the most recent, 

 there is a gradually increasing approximation to the existing 

 forms of nature : in the lowest, or eocene, there are only 

 three testaceous mollusks in the hundred identical with 

 existing species ; in the mid, or meiocene, there are nineteen ; 

 but in the uppermost, or pleiocene, the proportion is up- 

 wards of a half, or fifty-two per cent. During this latest 

 period, however, the mollusks continued to effect very con- 

 siderable deposits, less in extent, it may be admitted, than 

 in previous epochs when the secondary and carboniferous 

 limestones were formed, but still sufficient to vindicate their 

 claims to be, among animated beings, the most influential of 

 any, excepting zoophytes, in altering the relation between 

 sea and land. Examples of these deposits abound in the 

 north and south of Europe in general, in Asia and Africa, 

 and in New Holland ; but we select one only as an illustra- 

 tion of our position. 



In France, in the neighbourhood of Touraine, there is a 

 continuous bed of broken shells, of about nine ancient 

 square leagues in superficial extent, and at least twenty 

 feet in thickness : the whole mass of shells is estimated at 

 170 millions of cubic tons !f Such facts seem to warrant 

 the inference, that the living mollusks continue to be power- 

 ful cooperative agents in bringing about those changes 

 which slowly and imperceptibly are imprinting an altered 

 character on the features of our earth ; and it is so. In 

 Europe there are everywhere remarkable beds of shell-marl, 

 abounding with the remains of lacustrine mollusca, which 

 certainly constitute a very material part of its bulk ; and by 

 these depositions lakes and marshes have been filled up to 

 a great extent. Beaches composed of dead sea-shells are 



'* West of Eng. Journ. i. 3. "Conchology, studied in a logical manner 

 in its various . relations both to zoology and geology, may become a powerful 

 means of bringing this latter science to perfection. It is even allowable, 

 in the present day, to anticipate the time when Conchology shall arrive at 

 questions which relate to the general physics of the terrestrial globe, and 

 furnish us with the necessary materials for their solution." DESHAYES, Mag. 

 Nat. Hist. n. s. i. 9. 



t Malte'-Bnm's Geog. ii. 268 272. Jameson in Brewster's Edinb. 

 Encyclop. xv. 735. 



