366 Prof. J. Joly on the 



deformation, There is no apparent unconformity between 

 the Cherry Creek series and the gneissic rocks beneath. In 

 the Selkirk Range 40,000 feet of deposited rock are recorded, 

 but the correlation is somewhat obscure, suggesting that its 

 nge may not be entirely pre-Cambrian. In Nova Scotia 

 sedimentary rocks, probably Algonkian, amount to 26,000 feet. 

 The Canadian Huronian (equivalent to Algonkian) has been 

 estimated up to 50,000 feet. It is largely volcanic, and 

 contains unstratified igneous masses. 



It is remarkable that recent work has in many cases tended 

 to reduce the estimates of earlier observers. Chamberlin and 

 Salisbury (' Textbook of Geology,' p. 257) point out the 

 liability to over-estimation which exists in these cnses. 

 These same observers state ('Textbook of Geology/ p 192) 

 " The maximum thickness of the system (Keweenawan) has 

 been estimated as nearly 50,000 feet, but it is not impossible 

 that this estimate is an exaggerated one. If it be correct, 

 the Keweenawan is the thickest body of post-Archaean rock 

 referred to any one period. This seemingly incredible 

 thickness may merely mean inclined deposition and sub- 

 sequent tilting and shearing and the estimate be altogether 

 correct." And of the proterozoic systems collectively in the 

 Lake Superior region they write : — " If none of the estimates 

 are exaggerated, there is an aggregate of more than 30,000 

 feet of sedimentary rock in the proterozoic systems " (p. 198). 



It would appear then that the Keweenawan at its 

 maximum, so far as observed, is less than 50,000 feet, and 

 its true sedimentary thickness evidently considerably less. 

 The Huronian does not appear to have been reliably esti- 

 mated as above 15,000 feet. Together the maximum estimates 

 for the Algonkian are not above 60,000 to 65,000 feet, 

 inclusive of igneous materials. In its great development in 

 the Cordilleras it would appear that a maximum of 40,000 

 feet of true sediments would be safe, on the existing data. 



With the Archaean we are not here concerned. Van Hise 

 and Leith briefly summarize our knowledge of the earlier 

 rocks in these words : — " The Algonkian is characterized 

 by well-assorted fragmental and chemical sediments giving 

 evidence of extensive decomposition of land areas and of 

 the passage of normal cycles of erosion. Igneous rocks 

 are abundantly present, but for the most part are sub- 

 ordinate in amount to the sediments. The Archaean is 

 characterized mainly by igneous rocks with the sediments 

 in very small quantity. The Archaean sediments, moreover, 

 are frequently of 'wacke' type, and, so far as known, 

 are not largely of the cleanly assorted kinds resulting from 



