132 



HISTORICAL GEOLOGY. 



Fig. 134 (continued). 



Epochs and Sub-Epochs. 



Pleistocene, or Post-tertiary. 



Pliocene. 



Miocene. 



Eocene. 



f Upper or White 



Upper Cretaceous. ^ Chalk 



(Lower or Gray. 



Middle Cretaceous (Upper Green-Sand). 

 Lower Cretaceous (Lower Green-Sand). 



Wealden. 



Upper Oolite, j^*™ 1 ™ 



Middle OoHte.{g-»' r -^,a y : 



Lower Oe,i,e.{X^ lite 



Upper Lias. 

 Marlstone. 

 Lower Lias. 



Keuper. 



Muschelkalk. 



Bunter-sandstein. 



In the figures and maps introduced beyond, the numbers are used 

 as in the above tables : 1 standing for the Azoic ; 2 for the rocks of 

 the Potsdam period, 2 a for the Potsdam epoch, 2 b for the Calci- 

 ferous sand-rock ; 3 for rocks of the Trenton period, 3 a, 3 b, for the 

 epochs of this period ; and so on. 



The following map of the United States east of the Rocky Moun- 

 tains exhibits the geographical distribution of the rocks of the several 

 ages, — that is, the regions over which they are severally the surface- 

 rocks. 



The Silurian is distinguished by heavy horizontal lining ; and the 

 dotted line over the Silurian area divides the Upper Silurian (w) 

 from the Lower Silurian ( I) . 



