CHAPTER V. 

 THE CAMBRIAN PERIOD. 



Formations and Physical History. 



The great crustal movements which brought the Proterozoic era to 

 a close converted a large area within the limits of the North American 

 continent into land. This is shown by the geographic distribution 

 and the stratigraphic relations of the next succeeding system of 

 sedimentary rocks. This system, the Cambrian, 1 is the oldest system 

 of rocks now recognized as having been deposited during the Paleo- 

 zoic era. 



Where the base of the Cambrian system in North America is acces- 

 sible, the system is, as a rule, unconformable on underlying formations 

 (Figs. 88 and 89). Where this is the case, it is inferred that the sur- 



Figl 89. — Diagram representing the general relations between the Cambrian, €, and 

 the Proterozoic formations. The diagram is based on the general relations which 

 exist in the Lake Superior region. 



face on which it rests was a land surface and exposed to erosion for 

 a time before the formations above the unconformity were deposited. 

 Furthermore, the attitude of the Proterozoic strata is such as to show 

 that they were locally much disturbed prior to the notable erosion 

 which preceded the deposition of the Cambrian beds (Fig. 89). 

 Were the total area where the Cambrian system is unconformable 

 on its base known, it would furnish a basis for estimating the extent 

 of the withdrawal of the sea from the continent at the close of the 

 Proterozoic era, and before the deposition of the known Cambrian. If 

 it were possible, on the other hand, to determine the exact area once 

 covered by formations of the Cambrian system, this determination 



1 A summary of the literature on the North American Cambrian prior to 1892 

 is given by Walcott in Bull. 81, U. S. Geol. Surv. 



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