330 



THE CAMBRIAN. 



[BULL. 81. 



A comparison of the sections of the area on the eastern side of the 

 Rocky Mountains with that of the western sustains the view that dur- 

 ing late Algonkian?, Lower Cambrian, and Middle Cambrian time the 

 eastern Rocky Mountains and the plains to the eastward were above 

 the sea level, whilst to the west the sediments of the later Algonkian ?, 

 Lower Cambrian, and Middle Cambrian were accumulating west of the 

 line of the Wasatch Mountains and the extension of this line nearly 

 due north into British Columbia. Over all this region the coast line 

 and sea bed appear to have undergone a gradual depression during 

 early Cambrian time, and a relatively rapid depression at the begin- 

 ning of Middle Cambrian time. (Fig. 3.) 



THE INTERIOR CONTINENTAL PROVINCE. 



This province includes the Upper Mississippi area ; the eastern bor- 

 der or Adirondack sub- Province, the western border or Rocky Moun- 

 tain sub-Province, and the Llano County area of central Texas and 

 the Grand Canon of Arizona as a miuor southwestern sub-Province. 



The Lake Superior sandstones and the controversy in relation to 

 them will be noticed under a separate heading, after the description of 

 the recognized Upper Cambrian rocks of Wisconsin, Iowa, and Min- 

 nesota and their Canadian extension. 



UPPER MISSISSIPPI VALLEY. 



In the historical review the literature pertaining to these rocks has 

 been considered under the headings of u Wisconsin," u Minnesota," and 

 " Iowa," but as the strata all belong to one geologic basin it is consid- 

 ered best to speak of them collectively. m 



The formations that have been correlated with the Potsdam sand- 

 stone of New York will be treated of under two headings: (1) The 

 St. Croix or " Potsdam " sandstone proper, and (2) the sandstone of 

 the south shore of Lake Superior. 



The St. Croix ("Potsdam") sandstone. — The greater portion of the ex- 

 posures of the St. Croix or Lower sandstone occurs in Wisconsin, and it 

 here also presents many variations in sedimentation and fauna. Its 

 lower beds rest on the great pre-Cambrian nucleus of the center of the 

 State, extending about it from the Michigan line on the northeast to 

 where it passes into Minnesota on the northwest. Its line of contact 

 with the superjacent Magnesian limestone is also very extended owing 

 to the numerous streams that have cut through the limestone down into 

 the sandstone throughout its line of outcrop. The epoch was one of 

 accumulation of sand. The rocks are mainly light-colored sandstones 

 in central and southern Wisconsin, but embrace some beds of limestone 

 and shale. The maximum known thickness is about 1,000 feet. The 

 fauna is large and varied, and contains trilobites, gasteropods, ptero- 



