THE CAMBRIAN PERIOD 67 



between the Upper Cambrian and the succeeding Ordovician. 

 Thus we know that the Cambrian period closed quietly. As above 

 outlined, the one great physical event of the Cambrian was the 

 gradual submergence of large portions of the continent. 



hzzj es ^m turn 



GRCNVIUC. POTSDAM, JHtRt5» DRlfT. 



Fig. 31 

 Structure section in Saratoga County, New York, showing how Upper Cam- 

 brian strata overlap upon a hillock of pre-Cambrian rock (Grenville). 

 (After W. J. Miller, N. Y. State Mus. Bui 153.) 



The failure of any important stratigraphic break (uncon- 

 formity) between the Cambrian and Ordovician systems has 

 forced a rather arbitrary separation of these two systems, based 

 almost wholly upon important changes in organisms. 



Foreign Cambrian 



Europe. — Like that of North America, the Cambrian rocks 

 of Europe generally rest upon the profoundly eroded surface of 

 either Algonkian or Archean rocks. The physical geography of 

 the continent, however, differed considerably because the dis- 

 tribution of the rocks shows that the early Cambrian sea was almost 

 wholly limited to northern Europe, while the middle Cambrian 

 sea transgressed farthest over much of France, Germany, Bohemia, 

 Spain, and Sardinia, and by the beginning of the late Cambrian 

 the sea had retrogressed to occupy only northern Europe. Thus 

 much of Europe did not become progressively submerged like 

 North America. 



In Wales and Brittany the Cambrian strata appear to have a 

 maximum thickness variously estimated at from 12,000 to 20,000 

 feet, while in southern Sweden the whole Cambrian is only about 

 400 feet thick. Like those of North America, the rocks are mainly 

 clastic sediments of shallow water origin such as conglomerates, 



