AGE OF THE EARTH—JOLY. 298i 
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. Itis 
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 Archean 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 
