NO. I ABRUPT APPEARANCE OF THE CAMBRIAN FAUNA 3 



eluded in the Grand Canyon, Llano, and Belt series, and in forma- 

 tions correlated with them [see Van Hise and Leith, 1909, pp. 45, 

 46], were deposited in fresh- or brackish-water seas to which the 

 marine life of the extra-continental seas very rarely had access. 

 Such access occurred in mid-Beltian time when a protozoan, a 

 crustacean, and a few annelids penetrated and became adapted to 

 the conditions of the Montana sea, and more or less similar forms 

 to the Arizona sea. Other and difTerent forms may have lived in 

 these and other interior bodies of water, but as yet we have no 

 knowledge of them. 



On the eastern side of the continent the unconformity between the 

 oldest Cambrian and the known Algonkian rocks is so marked that 

 there is no question of a great stratigraphic and time break between 

 the two systems. The same is true of the Lake Superior, Hudson 

 Bay, and Cordilleran areas. The Algonkian sedimentary rocks of 

 the Atlantic coast region and the central interior continental areas, 

 like most, if not all, of those of the western interior or Cordilleran 

 areas, are of terrigenous origin, and in the absence of a marine fauna 

 are considered as having been deposited in epicontinental seas or 

 lakes of fresh or brackish water. 



PRE-CAMBRIAN ROCKS 



With our present information it appears that toward the close 

 of Archean time a period of diastrophism ensued, resulting in an 

 uplift of the North American, and probably other continental masses. 

 It was accompanied by local disturbances that resulted in the pro- 

 found folding and metamorphism of the Archean complex and the 

 formation of high mountains and uplands. Large areas of low lands 

 existed between the higher lands, and in these the terrigenous sedi- 

 ments began to accumulate in inland seas and lakes and in the 

 marine waters along the shores. Great quantities of eruptive matter 

 were extruded, the agencies of diastrophism continued to exert their 

 influence, but with decreasing energy, and during the latter part of 

 Algonkian time they were still less active. When the continental 

 area that had largely been a land surface since the first great uplift 

 at the close of the x\rchean began to admit the Cambrian sea, or 

 what is more probable, the sea began to rise, the latter found a sur- 

 face of relatively low relief, and in some districts great areas of 

 sediment that had been deposited in the inland lakes, so situated that 

 the Cambrian sediments were laid down almost conformably super- 

 jacent to them. 



