486 



GEOLOGY 



water, and therefore on a surface which was gradually depressed, 

 relative to sea-level, as the sediments were accumulated. The 

 greater proportion of limestone (chiefly dolomite) in the Upper 

 Cambrian of the southern and southeastern interior, points to 

 clearer seas, but perhaps not to deep ones. The adjacent lands 

 were perhaps too low to yield abundant sediment. Limestone is 

 also an important part of the Upper Cambrian of the Rocky Moun- 

 tains, 1 though clastic rocks predominate farther west. Where the 

 Upper Cambrian is limestone, it is not usually sharply differen- 

 tiated from the overlying Ordovician. 



Distribution and Outcrops of the Cambrian System 

 The Cambrian formations were once as wide-spread as the Cam- 

 brian seas themselves, but they are not now present over all the 

 area they once covered. The exposed edges of the strata have 

 suffered erosion, so that the border of the system as it now 

 appears about the areas of pre-Cambrian rock, is not its original 



Fig. 360. Diagram illustrating the relation of Cambrian formations, A, B, 

 and C, to older rocks. The diagram suggests that the Cambrian forma- 

 tions have been eroded back from their original margins, A', B f and C'. 



Fig. 361. Diagram illustrating the general relations of Cambrian beds in 

 the interior. The Cambrian, , is represented as appearing at the ex- 

 tremes of the diagram, and as dipping below younger beds between. 



border, and does not represent the shore-line of the Cambrian sea 

 when its waters were most wide-spread. Fig. 360 represents the 

 conditions which often exist. Each of the Cambrian formations, 

 represented by A, B, and C, formerly extended farther to the left. 

 1 Dawson, Bull. Geol. Soc. Am., Vol. XII, pp. 64-68. 



