STRUCTURE OF THE EARTH'S CRUST. 187 



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

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

 Vineyard, 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 coun- 

 tries in both hemispheres. 



We have thus more than forty distinct layers already 

 made out, each of which marks a distinct epoch in the 

 earth's history, indicating a more or less extensive and 

 important change in the condition of its surface. 



462. All the formations are not always found, or are not 

 developed to the same extent, in all places. It is the same 

 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 under- 

 stand how this must be so. Since irregularities in the 

 distribution of water upon the hard 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 continents and islands, as 

 the changes were multiplied. And since the rocks were 

 formed by the subsidence of sediment in water, no rocks 

 would be formed except in regions then covered by water; 

 they would be thickest at the parts where most sediment 

 was deposited, and gradually thin out towards their circum- 

 ference. 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. 



