IMPERFECTION OF THE GEOLOGICAL RECORD. 169 



fossilized, there are large groups which have only exceptionally left 

 traces of their existence : these are the animals which lived on land. 

 Fossil remains of land animals can only have survived when, during 

 great floods or inundations, or for some reason or other their carcasses 

 have been carried away by the water, floated hither and thither, and 

 been surrounded finally by hardening mud. This explains not only 

 the relative scarcity of fossil Mammalia, but also the fact that of 

 the most ancient Marsupials (Stonesfield slate), scarcely anything 

 is preserved but the underjaw, which, as the body decayed, was 

 easily detached, and, on account of its weight, offered most resist- 

 ance to the current of the water, and was the first part to sink to 

 the bottom. Although it has been shown by such remains that 

 Mammalia existed in the Jurassic period, yet the Eocene forms 

 are the first which give us an insight into the details of their 

 structure. 



Circumstances must have been more favourable to the preservation 



of fresh-water animals, and most of all to that of marine animals, 



since the marine deposits have a much greater extent than the 



locally confined fresh-water deposits. Thick formations seem in 



general to have arisen under one of two conditions : either in a very 



deep sea, protected from the operation of winds and waves, no 



matter whether the bottom was gradually rising or sinking in this 



case, however, the strata would be relatively poor in fossils, since 



only the inhabitants of the deep sea, which is comparatively wanting 



in animal and vegetable life, would be preserved or in a shallow sea, 



in which the bottom underwent a gradual and continued depression 



during long periods of time favourable to the development of a rich 



and varied fauna and flora. In this case the sea would have retained 



uninterruptedly its rich fauna so long as the gradual sinking of 



its bottom was counteracted by the continual supply of sediment 



deposited upon it. Thick formations, all or most of the strata of 



which are rich in fossils, must have been deposited on extended and 



very shallow regions of the sea, during a long period of gradual 



depression. 



Thus the great gaps which occur in the series of paleeontological 

 remains are explained by a consideration of the mode of origin of 

 deposits. These remains must necessarily be confined to the more 

 recent formations. The lower, more ancient, and very thick succes- 

 sions of strata in which the remains of the oldest fauna and flora 

 must have been buried, seem to have been so completely altered by 

 the heat of the molten interior of the earth, that the organic 



