THE CAMBRIAN PERIOD 65 



merged until the geographic conditions were much as depicted 

 upon the next paleogeographic map (Fig. 30). l The sea trans- 

 gressed northward over the great interior land to about the north- 

 ern border of the United States, the early Cambrian sounds merging 

 into this vast interior sea. Around Hudson Bay a large land 

 area still persisted, while Appalachia, and the lands along the 

 Pacific Coast, remained much as they were in the early Cambrian. 

 From Wyoming to Arizona a considerable area appears to have 

 remained as an island above even the latest Cambrian sea. The 

 northward trangression of this great interior sea is clearly estab- 

 lished by the fact that studies of actual outcrops and deep well 

 sections show successively younger and younger Cambrian sedi- 

 ments deposited by overlap northward upon the pre-Cambrian 

 rock surface. We also know that this interior sea was shallow 

 because of the character of the sediments which are very largely 

 clastic such as sandstones and shales often ripple marked, and with 

 conglomerates at the base. Some heavy limestone beds like those 

 in eastern New York, and between Virginia and Missouri, tell of 

 clearer, possibly deeper, water in those places. 



Close of the Cambrian. — The physical events above outlined 

 prove that the Cambrian period represents a long time, the best 

 estimates ranging from 2,000,000 to 3,000,000 years, though it 

 should be emphasized that we have no exact standard of compari- 

 son in years. The only object in presenting such figures is to im- 

 press upon the student the fact of the vast length of time involved. 

 Though the succeeding periods were by no means equal in duration, 

 the best estimates would make no one of them less than 1,000,000 

 years long. 



Throughout Cambrian time, and even at its close, North 

 America was not affected by any great physical disturbances such 

 as vulcanism, mountain making, or emergence of large areas of 

 land. Only locally, as for example in northern New York, and 

 possibly in the upper Mississippi Valley, is there any unconformity 



1 Recent studies seem to show that "near the close of Upper Cambrian 

 time a broad-spread movement resulted in the temporary withdrawal of the 

 Cambrian sea from the upper Mississippian (valley) area" (C. D. Walcott, 

 1914). According to Ulrich (Geol. Soc. Am. Bull, Vol. 23, pp. 627-647) the 

 time represented by this marine retrogression and immediately following 

 transgression should be called a separate period — the "Ozarkian." Strata 

 of this age would include some of the latest Cambrian and some of the 

 earliest Ordovician. 



