412 



HISTORICAL GEOLOGY. 



more numerous and more trenchantly distinct in the state of Xew York than 

 in most other parts of the continent ; and, moreover, the rocks of the state 

 were studied in detail and described by the geologists of the New York Sur- 

 vey between the years of 1836 to 1842, the systematizing period in geological 

 science. For these reasons, but especially the latter, a number of the sub- 

 divisions bear the names of New York localities. 



The following map of the United States east of the Rocky Mountains 

 exhibits the geographical distribution of the rocks of the several ages; 

 that is, the regions over which they are severally the surface-rocks. 



354. 



