188 GEOLOGICAL SUCCESSION OF ANIMALS. 



463. Each formation represents an immense period of 

 time, during which the earth was inhabited by successive 

 races of animals and plants, whose remains are often found, 

 in their natural position, in the places where they lived and 

 died, not scattered at random, though sometimes mixed to- 

 gether by currents of water, or other influences, subsequent 

 to the time of their interment. From the manner in which 

 the remains of various species are found associated in the 

 rock, it is easy to determine whether the animals to which 

 these remains belonged lived in the water, or on land, on the 

 beach or in the depths of the ocean, in a warm or in a cold 

 climate. They will be found associated in just the same 

 way as animals that live under similar influences at the 

 present day. 



464. In most geological formations, the number of spe- 

 cies of animals and plants found in any locality of given 

 extent, is not below that of the species now living in an 

 area of equal extent ; for though, in some deposits, the vari- 

 ety of the animals contained may be less, in others it is 

 greater than that on the present surface. Thus, the coarse 

 limestone in the neighborhood of Paris, which is only one 

 stage of the lower tertiary, contains not less than 1200 spe- 

 cies of shells ; whereas the species now living in the Mediter- 

 ranean do not amount to half that number. Similar relations 

 may be pointed out in America. Mr. Hall, one of the geolo- 

 gists of the New York Survey, has described, from the Tren- 

 ton limestone (one of the ten stages of the lower Silurian), 170 

 species of shells, a number almost equal to that of all the 

 species found actually living on the coast of Massachusetts. 



465. Nor was the number of individuals less than at 

 present. Whole rocks are entirely formed of animal re- 

 mains, particularly by corals and shells. So, also, coal is 

 composed of the remains of plants. If we consider the slow- 

 ness with which corals and shells are formed, it will give us 



