STRUCTURE OF THE EARTH'S CRUST. 395 



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. 



653. In most geological formations, the number of species 

 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 lime- 

 stone in the neighbourhood of Paris, which is only one stage of 

 the lower tertiary, contains not less than 1 200 species of shells ; 

 whereas the species now living in the Mediterranean do not 

 amount to half that number. Similar relations may be 

 pointed out in America. Mr. Hall, one of the geologists of 

 the New York Survey, has described, from the Trenton lime- 

 stone (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. 



654. Nor was the number of individuals less than at 

 present. Whole rocks are entirely formed of animal remains, 

 particularly of corals and shells. So, also, coal is composed 

 of the remains of plants. If we consider the slowness with 

 which corals and shells are formed, we may form some faint 

 notion of the vast series of ages that must have elapsed in 

 order to allow the formation of those rocks, and their regular 

 deposition, under the water, to so great a thickness. If, as 

 all things combine to prove, this deposition took place in 

 a slow and gradual manner in each formation, we must 

 conclude, that the successive species of animals found in them 

 followed each other at long intervals, and are not the work of 

 a single epoch, 



655. It was once believed that animals were successively 

 created in the order of their relative perfection ; so that the 

 most ancient formations contained only animals of the lowest 

 grade, such as the polyps and the echinoderms, to which 



