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:? 
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-Archzan 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:° 
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 Archean 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 oferosion. Igneous rocks are abundantly present, but for the most part are sub- 
_ ordinate in amount to the sediments. The Archzan is characterized mainly by igne- 
». ous rocks with the sediments in very small quantity. The Archzan 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.° 
According to the definition of Algonkian and Archean 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 Archean 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. 4 Loc. cit., pp. 21-22. 
2 Loc. cit., p. 192. 6 Textbook of Geology, vol. 2, p. 199. 
3 Loc. cit., p. 198. 
