Geologic Time. — Walcott. 367 
He draws from this the principle:— "The proper relative measure of 
geological periods is the maximum thickness of the strata formed dur- 
ing those periods."* 
In considering the time-ratios for the Paleozoic, Mesozoic and Ceno- 
zoic rocks of the North American continent as given by Dana and Wil- 
liams, I think that a too small proportion has been given to the Mesozoic 
and Cenozoic. In the Mesozoic of the western-central area occur the 
coal deposits of the Laramie series and the great development of lime- 
stones (from 10,000 to 20,000 feet) in the Cretaceous of Mexico. The 
limits of this paper do not permit a discussion of the available data 
bearing upon geologic time ratios; but from a comparison of the Paleo- 
zoic, Mesozoic and Cenozoic strata and the geologic phenomena accom- 
panying their deposition, I would increase the comparative length of the 
Mesozoic and Cenozoic periods so that the time-ratios would be: Paleo- 
zoic, 12; Mesozoic, 5; Cenozoic, including Pleistocene, 2. 
Duration of Post-Archean Geologic Time. 
Taking as a basis 17,500,000 years for Paleozoic time and the time- 
ratios, 12, 5 and 2, for Paleozoic, Mesozoic and Cenozoic (including 
Pleistocene), respectively, the Mesozoic is given a time duration of 
7,240,000 years; the Cenozoic, of 2,900,000 years; and the entire series of 
fossiliferous sedimentary rocks, of 27,650,000 years. To this there is to 
be added the period in which all of the sediments were deposited be- 
tween the basal crystalline Archean complex and the base of the Pale- 
ozoic. Notwithstanding the immense accumulation of mechanical sed- 
iments in this Algonkian time, with their unconformities and the great 
differentiation of life at the beginning of Paleozoic time, I am not will- 
ing with our present information to assign a greater time period than 
that of the Paleozoic, — or 17,500,000 years. Even this seems excessive. 
Adding to it the time period of the fossiliferous sedimentary rocks, the 
result is 45,150,000 years for post-Archean time. Of the duration of 
Archean or pre-Algonkian time I have no estimate based on a study of 
Archean strata to offer. If we assume Haughton's estimate of 33 per 
cent, for the Azoic period and 67 per cent, for the sedimentary rocks, 
Archean time would be represented by the period of 22,250,000 years. 
In estimating for the Archean, Haughton included a large series of 
strata that are now placed in the Algonkian of the Proterozoic of the 
U. S. Geological Survey; and 1 think that his estimate is more than 
one-half too large; if so, ten million years would be a fair estimate, or 
rather conjecture for Archean time. 
Period. Timk Di'h.vtion. 
Cenozoic, including Pleistocene 2,900,000 pears, 
Mesozoic 7,240,000 
Paleozoic 17,500,000 " 
Algonkian 17,600,000 " 
Archean 10,000,000(7) " 
It is easy to vary these results by assuming different values for area 
and rate of denudation, the rate of deposition of carbonate of lime, 
etc.; but there remains after each attempt I have made that was based 
*Nature, vol. 18, 1878, p. 268. 
