394 GEOLOGICAL SUCCESSION OF ANIMALS. 



8th. The Lower Tertiary, or Eocene, very abundant in the 

 Southern States of the Union, and to which belong the coarse 

 limestone of Paris, and the London clay in England. 



9th. The Upper Tertiary or Miocene, and Pleiocene, found 

 also in the United States, as far north as Martha's Vineyard, 

 and Nantucket, and very extensive in Southern Europe, as well 

 as in South America. 



10th. The Drift, forming the most superficial deposits, and 

 extending over a large portion of the northern countries in 

 both hemispheres. 



\Ve have thus more than forty distinct layers already made 

 out, each of which marks a distinct epoch in the earth's his- 

 tory, indicating a more or less extensive and important change 

 in the condition of its surface. 



651. All the formations are not everywhere found, or are 

 not developed to the same extent, in all places. So it is 

 with the several strata of which they are composed. In other 

 words, the layers of the earth's crust are not continuous 

 throughout, like the coats of an onion. There is no place on 

 the globe where, if it were possible to bore down to its centre, 

 all the strata would be found. It is easy to understand how 

 this must be so. Since irregularities in the distribution 

 of water upon the solid crust have, necessarily, always existed 

 to a certain extent, portions of the earth's surface must have 

 been left dry at every epoch of its history, gradually forming 

 large islands and continents, as the changes were multiplied. 

 And since the rocks were formed by the subsidence of sedi- 

 ment in water, no rocks would be formed except in regions 

 covered by water ; they would be thickest at the parts where 

 most sediment was deposited, and gradually thin out to- 

 wards their circumference. We may therefore infer, that all 

 those portions of the earth's surface which are destitute of a 

 certain formation were dry land, during that epoch of the 

 earth's history to which such formation relates, excepting, 

 indeed, where the rocks have been subsequently removed by 

 the denuding action of water or other causes. 



652. 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- 



