AGE OF THE EARTH JOLY. 279 



Creek series and the gneissic rocks beneath. In the Selkirk Range 

 40,000 feet of deposited rock are recorded, but the correlation is some- 

 what obscure, suggesting that its age may not be entirely pre-Cam- 

 brian. In Nova Scotia sedimentary rocks, probably Algonkian, 

 amount to 26,000 feet. The Canadian Huronian (equivalent to Algon- 

 kian) 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 * 

 point out the liability to overestimation which exists in these cases. 

 These same observers state: 2 



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 subsequent tilting and shearing and the estimate be altogether correct. 



And of the proterozoic systems collectively in the Lake Superior 

 region they write : 3 



If none of the estimates are exaggerated, there is an aggregate of more than 30,000 feet 

 of sedimentary rock in the proterozoic systems. 



It would appear, then, that the Keweenawan at its maximum, so far 

 as observed, is less than 50,000 feet, and its true sedimentary thick- 

 ness evidently considerably less. The Huronian does not appear to 

 have been reliably estimated as above 15,000 feet. Together the max- 

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

 inclusive of igneous materials. In its great development in the Cor- 

 dilleras it would appear that a maximum of 40,000 feet of true sedi- 

 ments 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 : 4 



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 igne- 

 ous rocks with the sediments in very small quantity. The Archaean sediments, more- 

 over, are frequently of wacke type, and, so far as known, are not largely of the cleanly 

 assorted kinds resulting from complete decomposition as in the Algonkian. 



Similar testimony is borne by Chamberlin and Salisbury. 5 

 According to the definition of Algonkian and Archaean we must 

 draw a line at the base of the former as representing that limit at 

 which geological time, as an era of sedimentation and solvent denuda- 

 tion, began. "The Archaean was essentially a period of world-wide 

 vulcanism, and in the relative proportions of rocks of igneous and sedi- 



i Textbook of Geology, p. 257. * Loc. cit., pp. 21-22. 



a Loe. cit., p. 192. & Textbook of Geology, vol. 2, p. 199. 



3 Loc. cit., p. 198. 



