NO. I ABRUPT APPEARANCE OF THE CAMBRIAN FAUNA 5 



26], include in the Belt Mountain section 4,400 feet of limestone and 

 7.600 feet of arenaceous and siliceous strata [Walcott, 1899, p. 204], 

 and in the Mission Range section about 6,000 feet of calcareous strata 

 and over 18,000 feet of arenaceous and siliceous beds [Walcott, 

 1906, p. 18]. 



In the Lake Superior region the Keweenawan system is made 

 up of a great series of mainly igneous rocks in the lower portion, 

 with sedimentary sandstone and siliceous strata above, that form 

 about 15,000 feet of the 50,000 feet in thickness provisionally as- 

 signed to this system [Chamberlin and Salisbury, 1906, p. 192]. 



The eastern or Atlantic Province Algonkian strata are represented 

 in Newfoundland by about 10,000 feet of mainly siliceous sediments, 

 and in Nova Scotia there may be 15,000 feet of similar sediments 

 [Chamberlin and Salisbury, 1906, p. 204]. 



Sources of Sediments. — Of the sources of the sediments of the 

 Algonkian (Proterozoic) formations of the Lake Superior region, 

 Chamberlin and Salisbury consider [1906, Vol. 2, p. 199] "that a 

 large portion of the sediments was produced by mature decomposi- 

 tion of older rocks, and this implies that they were not derived by 

 rapid mechanical abrasion such as that which accompanies and fol- 

 lows great elevation and excessive precipitation. The great series of 

 quartzites were derived from the complete decomposition of quartz- 

 bearing rocks, and involved the almost complete separation of the 

 quartz grains from other constituents, while the thick beds of shale 

 arose from the complementary clayey products of decomposition from 

 which most of the basic oxides had been removed by carbonation." 

 These authors consider that the sediments were deposited on the bed 

 of the Algonkian sea [p. 200]. 



The sources of the sediments of the Algonkian formations of the 

 Cordilleran and Atlantic Provinces appear to have been similar to 

 those described above, and all indicate relatively low elevations and 

 quiet conditions of deposition except in the case of the massive, 

 coarse sandstones in the lower Belt series of northwestern Montana 

 [Walcott, 1906, p. 26]. 



The Algonkian formations of the Belt, Grand Canyon, Llano, and 

 Avalon series all contain a large amount of bluish-black, red, and 

 green, finely arenaceous, siliceous shales. These often form beds 

 hundreds of feet in thickness, and extend over wide areas. With 

 the exception of Cryptocoan f occidental e Dawson [Walcott, 1906, 

 p. 18] in some of the interbedded limestones there is no evidence of 

 the presence of life in them. 



