332 



GEOLOGIC TIME. 



The Rev. Samuel Hou.uiitou obtained the following time ratios from 

 the niaxinmni thickness of strata as they occur in Europe: 



Scale of (jeolo(/ical time. 



From theory of 

 cool i no- globe. 



I Per cent. 



Azoic 33-0 



Paleozoic -tl 



Neozoic 20 



Total ! lOU-O 



From iiiaxinium 



thickuess of 



strata. 



Per cent. 

 34-3 

 42-5 

 23 '2 



100-0 



He draws from this the principle: " The proper reJatire measure of 

 geological periods is the m<i.rl)num thlel'iiess of the strata formed during 

 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 

 Williams, I think that a too small proi)ortion 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 limestones (from 10,000 to 20,000 feet) in the Cretaceous of Mexico. 

 The limits of this paper do not ])ermit of a discussion of the available 

 data bearing upon geologic time ratios; but from a comparison of the 

 Paleozoic, Mesozoic, and Cenozoic strata and the geologic phenomena 

 accompanying their deposition, I would increase the comparative length 

 of the Mesozoic and Cenozoic periods so that the time ratios would be : 

 Paleozoic, 12; Mesozoic, 5; Cenozoic, including Pleistocene, 2. 



DURATION OF POST-ARCHEAN (rEOLOdlC 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), res])ectively, the Mesozoic is given a time duration of 

 7,240,000 years; the Cenozoic, of 2,000,000 years, and the entire series 

 of fossiliferous sedimentary rocks, of 27,6r)O,00O years. To this there 

 is to be added the period in which all of the sediments were deposited 

 between the basal crystalline Archean complex and the base of the 

 Paleozoic, Notwithstanding the immense accumulation of mechanical 

 sediments in this Algonkian time, with their unconformities, and the 

 great dififerentiation of life at the beginning of Paleozoic time, I am not 

 willing 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 



Nature, July 4. 1878, vol. xviii, p. 268. 



