220 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 mingled 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 are 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 and of a similar character ; for though, 

 in some deposits, the variety 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 species of shells ; whereas the species now 

 living in the Mediterranean do not amount to half that num- 

 ber. Similar relations may be pointed out in America. 

 Mr. Hall, one of the geologists of the New York Survey, has 

 described, from the Trenton 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 now 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 of corals and shells. So, also, coal is 

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



