Review of Recent Geological Literature. 413 
area traversed by Prof. Culver, he gives very interesting details of the 
glacial drift and of an ice-dammed lake. Within thirty miles southward 
from the 49th parallel, ice was accumulated so thickly west of the main 
eastern range of the Rocky mountains that it outflowed eastward through 
the passes, carrying diorite boulders from ledges west of the watershed 
to a distance of several miles on the plains at the eastern base of the 
mountains. No Laurentian drift was observed there, but in the valley 
at the head of St. Mary's river, a tributary of the Belly river, on lati- 
tude 113" 30', five to twenty miles south of the international boundary, 
shore lines of a glacial lake, which was probably formed by the neigh- 
boring barrier of the Laurentide ice-sheet on the northeast, occur up to 
the hight of at least 800 feet above the present St. Mary's lakes, or ap- 
proximately 5,400 feet above the sea. 
Estimates of Geologic Time. By Warren Upham. Am. Jour. Sci., 
Ill, vol. xlv, pp. 209--320; March, 1893. Recent estimates of the age of 
the earth vary widely, but this paper shows that the more reliable geol- 
ogic measurements and ratios lie inside the probable limit of 100 million 
years assigned from physical data by Sir William Thomson (now Lord 
Kelvin). Computations by Wallace, based on the rates of land erosion 
and consequent oceanic sedimentation, allow 28 million years for the 
deposition of all the sedimentary rock strata. Mr. Upham, however, 
shows that by certain reasonable changes in the premises of this com- 
putation the result would be three times as great or 84 million years. 
Another method of reaching an estimate is found in the ratios of the 
Recent and Glacial periods to the preceding and far longer geologic eras. 
The mean of numerous independent measurements of the probable 
length of the Postglacial or Recent period, from the departure of the ice- 
sheets to the present time, is 8,000 years; and for the duration of the 
Glacial period the writer accepts Prestwich's opinion, that the Ice age 
in reaching its culmination occupied 15,000 to 25,000 years and in waning 
continued onward perhaps 8,000 to 10,000 years or less. Further, from 
the rate of extinction and new appearance of marine molluscan species, 
the proportion of Glacial and Recent time to that since the beginning 
of the Tertiary era is believed to be as one to fifty or a hundred. 
Thence, according to closely agreeing ratios deduced from comparison 
of the thicknesses of the rock strata by Dana, Alexander Winchell, and 
Davis, the lengths of the successive eras since the Archasan are esti- 
mated to have been, for the Paleozoic, 36,000,000 years; the Mesozoic, 
9,000,000; and the Cenozoic, comprising both Tertiary and Quaternary 
time, 3,000,000. In the Quaternary or present era Mr.Upham would in- 
clude not only the Ice age and subsequent time but also a somewhat 
long time of preglacial increasing high uplift of the areas that became 
finally ice-enveloped, giving to tiie Quaternary in all probably 100,000 
years, or about a thirtieth part of the entire Cenozoic. 
The Olacial Sitccession in Ohio. By Frank Leverett. Journal of 
Geology, vol. i, pp. 129-146, with map; Feb.-March, 1893. Ten approxi- 
mately parallel and successively formed terminal moraines of the Mau- 
