HISTORICAL GEOLOGY. 



Fig. 201 (continued). 



143 



Epochs. 



S Upper or White 

 Chalk. 

 Lower or Gray. 



Middle Cretaceous (Upper Green-sand). 

 Lower Cretaceous (Lower Green-sand). 



Upp«r051 J U.{ru^*,Por S ,. n d 



Mida,eOo, y ,e.{g« ay . 



t „„„. n*i„f^ f Stonesfield. 

 Lower0o, y t6 -{ Inferior Oolyte. 

 Upper Lias. 

 Marlstone. 

 Lower Lias. 



Keuper. 



Muschelkalk. 



Ss Bunter-sandstein. 



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

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

 of the Primordial, 3 for the rocks of the Canadian period ; 3 a, 3 b, 



3 c, for its subdivisions ; 4 for rocks of the Trenton period, 4 a, 4 b, 



4 c, 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. 



The Devonian, by heavy vertical lines. 



The Carboniferous, by light cross-lines on a black ground, or by a 

 black surface, or by dots on a black ground (the first the Subcar- 

 boniferous, the second the Coal- formation, the third the Permian). 

 The black areas are the Coal-areas of the country. 



