STRUCTURE OF THE EARTH'S CRUST. 219 



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. 



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 every where 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 under- 

 stand 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 sediment 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 towards 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. 



