THE CAMBRIAN PERIOD. 225 



are generally conformable. 1 Where it rests on pre-Cambrian forma- 

 tions, as in the States mentioned above, it is in unconformity with 

 them. (Figs. 88 and 89.) 



The Upper Cambrian. 

 In the Later Cambrian (Potsdam, Saratogan) epoch, the Appala- 

 chian region on the one hand, and the Cordilleran region on the other, 

 were still submerged, as shown by the conformable (in general) super- 

 position of the Upper Cambrian beds on the Middle Cambrian, in both 



Fig. 9.4. — This figure represents a section in the Menominee region of northern Michi- 

 gan, and shows the Potsdam sandstone. €s, in unconformity with Proterozoic 

 formations, A. At, Archean. (Van Hise, U. S. Geol. SurvO 



regions. In addition, the Upper Cambrian beds are spread widely 

 over the interior of the continent, indicating that the sea had extended 

 itself greatly at the expense of the land in the Potsdam epoch. Where 

 the Upper Cambrian rests on pre-Cambrian formations, it is in uncon- 

 formity with them. 



Fig. 95 shows something of the distribution of the Upper Cambrian 

 series. The black areas represent the areas where the series comes 

 to the surface; the areas lined horizontally represent the areas where 

 it is confidently believed to be present, though buried; the areas shaded 

 by dashes are the areas where the series is probably present; while 

 the dotted tracts represent the areas from which the series is thought 

 to have been removed by erosion. The blank areas, except in Mexico 

 and Central America, stand for the tracts which are thought to have 

 remained above the sea throughout the Cambrian period; 2 in those 

 countries, they represent lack of knowledge. 



Sections of the Cambrian and their Interpretation. 



Fig. 96 represents known sections of the Cambrian formations 

 in various localities. 3 The study of these sections, while involving 



1 Ulrieh and Schuchert think that the Appalachian synclinorium of Early Cam- 

 brian time was largely drained at the close of that epoch. Loc. cit., p. 636. 



2 Since this map was prepared, Upper Cambrian beds have been reported from 

 southwestern Texas. Richardson, Reconnaissance in Trans Pecos, Texas, Bull. 9. 

 Univ. of Tex. Min. Surv. 



3 Walcott, Bull. 81, U. S. Geol. Surv. 



