FOSSIL CONCHOLOGY. 



239 



day different species inhabiting different latitudes ; and difference of 

 temperature in the waters of different lakes in the same latitude, 

 might occasion a great change in the character of the inhabitants. 

 The consideration that the value of the evidence from organic re- 

 mains, was originally derived from the evidence of position, and 

 must ever remain, more or less, dependent upon it, appears to have 

 escaped the attention of many geologists, exclusively attached to the 

 study of zoological characters. Among our ingenious neighbours, 

 the French, perhaps too ready to form generalizations from a limited 

 number of facts, the value of the evidence to be derived from the 

 study of fossil conchology is greatly overrated, when they would 

 make it independent of position or gisement. Could the most sci- 

 entific conchologist or naturalist have discovered from the organic 

 remains in the Wealden beds, whether they were deposited before or 

 after the green sand ? Certainly not. He might have ascertained 

 that they were fresh water, and not marine beds ; but this would not 

 have assisted him in discovering their relative age. Fortunately, we 

 have here the evidence of superposition ; for the green sand lies over 

 the upper Wealden beds, and, therefore, is a later deposition. When 

 the different periods of time in which different species of animals 

 first appeared in different latitudes, shall be known, then, and not till 

 then, can we predicate with certainty respecting the relative age of 

 strata from their organic remains alone. 



I shall now proceed to state the rules attempted to be established 

 for determining the relative ages of the tertiary strata by organic 

 remains. 



M. Deshayes considers, that the relative ages of different groups 

 of strata or formations may be determined, by their zoological char- 

 acters alone ; that is, by the species of shells they contain. He 

 forms two grand divisions of stratified formations : — 



1. Those which contain no species of shells analogous to exist- 



ing species.'^ 



This division is stated to comprise all the secondary strata. 



2. Strata which contain a greater or less number of species analo- 



gous to existing species. 



The last division comprises all the tertiary formations. Again he 

 subdivides this division into three groups, according to the greater 

 or less proportion of species of shells, that they each contain anal- 

 ogous to living species* 



In the more ancient group he places the tertiary formations of the 

 Paris basin, the London basin, the Isle of Wight, and of a part of 

 Belgium, a small part of the Gironde, and the tertiary strata of the 

 Vicentin. 



* By esphe analogue, M. Deshayes means identical species. 



