EVIDENCES OF PRIMITIVE LIFE WALCOTT. 239 



On the eastern side of North America the rocks are mostly formed 

 of siliceous mud and sand; in the Lake Superior region, siliceous 

 mud, sand, gravel, and an immense mass of eruptive rock; in the 

 Rocky Mountain and adjacent areas, siliceous and calcareous muds, 

 fine sands, and a small amount of eruptive rock. In Montana the 

 Algonkian rocks are from 12,000 to 25,000 or more feet in thickness 

 and contain gi'eat beds of limestone, in which traces of life have been 

 found. One of them, called the Newland limestone, is particularly 

 rich in algal deposits.^ 



UNCONFORMITY BETWEEN THE CAMBRIAN AND PRE-CAMBRIAN 



ROCKS.' 



The variation in thickness of the basal Cambrian conglomerate 

 seems to indicate that the pre-Cambrian surface over which it was 

 deposited was broadly irregular. The Cambrian sea was evidently 

 transgressing across the dark siliceous shales of the pre-Cambrian 

 land and reducing them to rolled pebbles, angular fragments, and 

 mud. The mud gave origin to small lentiles of shale similar in 

 character to the shale below the unconformity, while lentiles of 

 sandstone of greenish tint indicate that fine material was being de- 

 rived from still older pre-Cambrian formations than the shale. 



Of greater importance is the evidence that the sediments of the 

 tAvo periods were deposited under different physical conditions. The 

 Cambrian sandstones are composed of clean, well- washed grains, and 

 the. Cambrian calcareous and argillaceous shales were deposited as 

 muds offshore along with the remains of an abundant marine life. 

 The Algonkian Hector shales^ of the pre-Cambrian are siliceous 

 and without traces of life; the sandstones are impure and dirty, 

 Avith the quartz grains a dead milky white or glassy and iron 

 stained. These sediments were evidently deposited in relatively 

 quiet, muddy, fresh or brackish waters. 



I do not compare the limestone formations, as in the Cambrian 

 they are 2,000 feet or more above the plane of unconformity at the 

 base of the Cambrian and much farther below in the Algonkian 

 series. 



ORIGIN OP ALGONKIAN LIMESTONES.* 



The stream flow and drainage into the Algonkian lakes undoubt- 

 edly afforded all of the soluble mineral matter necessary to account 

 for the limestones, siliceous shales, and calcium carbonate deposits of 



1 Pre-Cambrian Algonkian algal flora, Smithsonian Misc. Coll.. vol. 64, no. 2, 1914. 



= Pre-Cambrian rocks of the Bow River Valley, Alberta, Canada, Smithsonian Misc. 

 Coll., vol. 53, no. 7, 1910, p. 426. 



3 Algonkian, Hector-Corral Creek scries. Bow River Valley, Alberta, Canada. See sec- 

 tion, Smithsonian Misc. Coll., vol. 53, no. 7, 1910, p. 428. 



* Pre-Cambrian Algonkian algal flora, Smithsonian Misc. Coll., vol. 64, no. 2, 1914. p. 84, 



