﻿156 G. D. Walcott — Cambrian System of North America. 





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were the shore line of the pre-Cambrian Keweenaw sea. What 

 the eastern boundaries of this sea were, we do not now know, 

 but the inference, from what is known 

 of the Archean of the Appalachian 

 system, is that portions of the latter 

 were above the ocean during the depo- 

 sition of the Keweenaw system. 



The traces we now have of this Ke- 

 weenaw land point to its extension 

 from the Lake Superior region south 

 to Central Texas and westward to 

 Central Northern Arizona. A glance 

 at the map (fig. 8) shows how far 

 apart the relatively small exposures 

 are ; but, the great similarity of the 

 sections and their position in relation 

 to the Upper Cambrian that rests on 

 the eroded surface of each visible area, 

 points to a wide spread orographic 

 movement that raised the entire cen- 

 tral portion of the Continent and again 

 depressed it at the termination of the 

 period of erosion preceding the deposi- 

 tion of the Upper Cambrian or Pots- 

 dam sediments of the Upper Missis- 

 sippi Valley, Central Texas and Ari- 

 zona. 



The existence of such a land over 

 the area mentioned, is shown by the 

 sections we now know ; and I think 

 that, when the areas of Cambrian and 

 Archean rocks in Missouri and also 

 along the Southern Appalachian chain 

 come to be studied with the view that 

 such a land existed during the period 

 of the deposition of the earlier deposits 

 of the Cambrian system, evidence will 

 be forthcoming to show its former 

 presence over a large area. On the 

 north it probably joined the Archean 

 continent and thus gave a greater ex- 

 tension of the pre-Cambrian continent 

 to the south that, during the early 

 history of the Cambrian period, fur- 

 ished more or less of the sediments of 

 the strata of the Lower and Middle Cambrian. The Archean 

 boundaries of the Keweenaw sea continued after the elevation 



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