250 



GEOLOGY. 



In other words, the structure of the formation confirms the conclusion 

 drawn from the distribution of the Upper Cambrian, that it was 

 deposited in an advancing sea. 



The greater portion of limestone, chiefly dolomite, in the Upper 

 Cambrian series of the southern and southeastern interior, points to 

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

 were perhaps too low to yield abundant sediment. It has been con- 



L&' 



Fig. 99. — Cross-bedded Potsdam sandstone, Dells of the Wisconsin. (Blackwelder.) 



jectured, but not proved, that the limestone of the interior was largely 

 precipitated from solution. 1 Limestone is also an important part of 

 the Upper Cambrian of the Rocky Mountains, 2 though clastic rocks 

 predominate farther west. Where the Upper Cambrian is limestone, 

 it is not usually sharply differentiated from the overlying Ordovician. 

 This is the case in the southern Appalachians, 3 as well as in Indian 

 Territory, Missouri, 4 and Nevada. 5 



Variation in a given locality. — The variations in the conditions 

 of sedimentation in the same region are well illustrated by the sequence 



1 TJlrich and Sehuchert, loc. cit., p. 637. 



2 Dawson, Bull. Geol. Soc. Am., Vol. XII, pp. 64-8. 



3 Folios U. S. Geol. Surv.; also Prosser, Jour. Geol., Vol. VIII, p. 662. 



4 Keyes, Am. Geol. , Vol. XXIX, pp. 384-7. 



5 King, Geological Exploration of the Fortieth Parallel, Vol. I. 



