AGE OF THE EARTH — JOLY. 281 



series — which again is divided by unconformities — is a significant 

 feature observed in many widely separated parts of the world. 



The above cited facts seem to show (1) that we are entitled to com- 

 mence our reckoning of the sedimentary column at the base of the 

 Algonkian; (2) that the existing sedimentary deposits of that epoch 

 are probably not greater than the more or less concordant observa- 

 tions from several localities indicate; (3) that the early sedimentation 

 was similar in character to that which proceeded in subsequent periods. 



Although much is gained by these deductions, it is difficult to deter- 

 mine any approximate time equivalent for these ancient deposits. It is 

 true that there is no reason to suppose that their derivation proceeded 

 at a different rate from more recent ones; their rate of accumu- 

 lation, however, may have been and indeed probably was quickened 

 by less stable crust conditions, permitting more localized depressions 

 and greater concentration. The geographical disposition of the earlier 

 sediments sometimes affords evidence of this. There are, again, 

 several unconformities in the pre-Cambrian succession which do not 

 appear to be represented in the known sedimentary accumulations. 

 Van Hise and Leith recognize the principal unconformity as separat- 

 ing the Archaean from the Algonkian. Adams, however, recognizes 

 one of equal significance beneath the Upper Huronian. Three uncon- 

 formities occur within the Algonkian. That these are indicative of 

 considerable lapses of geological time is highly probable. 



A discussion of the time allowance for these early unconformities 

 would lead us too far into speculation. It may be observed, however, 

 as regards the evidence for prolonged periods of denudation deduced 

 from regional base leveling, that the instability of the early crust 

 must again be kept in mind. It is probable that the Algonkian 

 mountains were not of the dimensions of those of later periods and 

 that, therefore, they were at once more rapidly formed and more 

 rapidly removed. Van Hise and Leith suggest that the unconformi- 

 ties may represent as much sediment again as now remains to observa- 

 tion. This, of course, can only be matter of opinion; and I have as 

 far as possible endeavored to exclude what is purely matter of opinion 

 from this review of the subject. It would seem, however, that Sollas's 

 estimate of 82,000 feet of sediment includes such an allowance as ap- 

 pears possible to Van Hise and Leith. 



Taking all into account — and much has been omitted which might 

 be said upon the subject — it does not appear that Prof. Sollas's com- 

 pilation of the stratigraphical column need be seriously disturbed. 

 If we double the estimate for the Jurassic we at least tend to reduce 

 the possibility of error of deficiency in the thickness assigned to this 

 system. This brings the column up to, say, 345,000 feet. 



What now, finally, is the time value of this enormous total ? Un- 

 fortunately the average rate of collection is a very indeterminate 



