MODE OF PRESERVATION 345 



the reptiles, mammals, and, more rarely, birds of the land, 

 mingled with those of the crocodiles, turtles, and fishes that lived 

 in the water. Similar lake-beds are known in other continents. 



It is on the sea-bed that the conditions are most favourable 

 to the preservation of the greatest number and variety of fossils. 

 Among the littoral deposits ground by the ceaseless action of the 

 surf, fossils are not likely to be abundant or well preserved, but 

 in quieter and deeper waters vast numbers of dead shells and the 

 like accumulate and are buried in sediments. The fossils are 

 not, however, uniformly distributed over the sea-bottom ; in some 

 places they are crowded together in multitudes, while large areas 

 will be almost devoid of them. The differences are due to variations 

 in temperature, in the character of the bottom, in food supply, 

 and other conditions. In tropical seas, swept by currents which 

 bring in abundant supplies of food, the luxuriance of life is wonder- 

 ful, and in such places a correspondingly large number of fossils 

 are preserved. However, even under the most favourable circum- 

 stances, the fossils can never represent more than a fraction of the 

 life of their times. Indeed, the wonder is that so much of the 

 life systems of past ages has been preserved, rather than that so 

 large a part has been irretrievably lost. 



The ways in which fossils are preserved vary much, according to 

 circumstances, but three groups include all the principal kinds. 



(1) Preservation of more or- less of the original substance. 

 In certain rare instances an organism may be preserved intact, 

 as have been the carcases of the extinct species of elephant and 

 rhinoceros which are found in the frozen gravels of Siberia, 

 which after thousands of. years of burial are still eagerly devoured 

 by the wolves. Much more common is the decomposition of 

 the soft structures and the preservation of the hard parts, — 

 bones, shells, etc. Most of the shells and bones found in the 

 rocks of later geological date arc composed of the material origi- 

 nally belonging to them, though they have suffered much loss 

 of substance. The carbon of coal plants is that which was 

 present in the living vegetation, but the volatile matters have 

 disappeared. 



