578 EDWARD STEIDl^MANN 



The unconformity is shown by the variable thickness of the 

 qnartzite- slate series, slight discordance of bedding, and a basal 

 conglomerate. 



The Algonkian system consists of alternating quartzites, slates, 

 and conglomerates, which are variable from place to place; and 

 show cross-bedding, ripple marks, and mud cracks. The materials 

 are not well assorted, and in the sandy beds, the prevailing colors 

 are shades of red; green predominates in the shales. Fossils and 

 limestone appear to be lacking. Blackwelder is inclined to view 

 these sediments as subaerial, largely fluviatile deposits. 



Cross and Hole' state that the pre-Cambrian in the Engineer 

 Moimtain quadrangle of Colorado include Archaean gneisses and 

 schists derived from granites, diorites, and diabase. These are 

 intruded by granitic and gabbroic masses. Several thousand feet 

 of massive and some thin-bedded quartzites, white, brown, and 

 black, and occasional bands of brown to black shale, all of Algonkian 

 age, overlie the Archaean. 



Diller^ describes the occurrence of asbestos in the pre-Cambrian 

 rocks near Casper, Wyo., and the Grand Canyon of Arizona. At 

 Casper, Wyo., the asbestos occurs mostly at the igneous contact 

 between serpentine and granite. In the Grand Canyon, asbestos 

 occurs only where diabase sills have been intruded into Algonkian 

 magnesian limestones. 



Diller'^ states that the rocks of the asbestos area south and south- 

 east of Casper, Wyo., are hornblende schist, diorite, granite, and 

 serpentine of pre-Cambrian age. 



In the Grand Canyon of Arizona, certain Algonkian limestone 

 beds contain asbestos where they have been intruded by diabase sills. 



Daly* finds that the great stratigraphic problem of the Shuswap 

 Lakes region is the correlation of the non-fossiliferous, crystalline 



^W. Cross and A. H. Hole, "Engineer ilountain Folio," U.S.G.S. Folio 171, 

 1910, 14 pp., 5 pis., I lig. 



-J. S. Diller, "The Types and Modes of Occurrence of Asbestos in the United 

 States," Jour. Can. Miii. Inst., 1911, pp. 93-107. 



3 J. S. Diller, "The Tj-pes, Modes of Occurrence, and Important Deposits of 

 Asbestos in the United States," Bull. 470, U.S. Geological Survey, 1911, pp. 505-23. 



^ R. A. Dah', "Reconnaissance of the Shuswap Lakes and Vicinity (South Central 

 British Columbia), Canada Department of Mines, Ottawa, 1912. 



