10 USES OF FOSSILS IN GEOLOGY. 



sufficient to identify the two deposits as having been formed 

 within the same general period, although not sufficient to 

 say that they are absolutely synchronous. Then again 

 there is a distribution of organisms according to the depth 

 of the sea, and the nature of the sea-bottom, so that the 

 fauna of a deep-water formation will necessarily be dif- 

 ferent from that of a shallow-water one, and that of a 

 sandstone different from that of a clay. 



In addition to their chronological value, fossils are 

 also important in indicating the conditions under which 

 the formations were deposited. In the case of the later 

 beds, where most of the genera are still existing, it is easy 

 to distinguish a marine deposit from a freshwater or 

 terrestrial one. And even in the earlier, where most of 

 the genera are extinct, it is not very difficult, since many 

 of the groups of animals still have representatives at the 

 present day, and these in some cases are entirely marine, 

 as for instance the corals, echinoderms, and cephalopods. 

 Then again none of the freshwater lamellibranchs possess 

 one adductor muscle only; and very few of the land or 

 freshwater gasteropods are siphonostomatous. 



Fossils, especially plants, also furnish some evidence of 

 the climate of past times. But the evidence afforded by 

 marine animals is of comparatively little value, except 

 when we are dealing with existing species. This is owing 

 to the fact that at the present day the individual species 

 of the same genus have often a very different range in 

 space, some being tropical forms, others polar. But even 

 in the case of extinct species, the assemblage of the 

 genera is sometimes such as characterises some region at 

 the present day, as for instance in the tropical mollusks 

 of the London Clay. 



The depth of the sea in which a formation was deposited 



