674 



THE JOURNAL OF GEOLOGY. 



giving the standard time-unit used the name geochrone. The 

 geochrone used by him in obtaining a standard scale of geochron- 

 ology is the period represented by the Eocene. His time-scale 

 gives 15 for the Paleozoic; 3 for the Mesozoic ; and 1 for the 

 Cenozoic, including the Quaternary and the Recent. 1 



The Rev. Samuel Haughton obtained the following time- 

 ratios from the maximum thickness of strata as they occur in 

 Europe : 



SCALE OF GEOLOGICAL TIME. 



He draws from this the principle — " The proper relative meas- 

 ure of geological periods is the maximum thickness of the strata 

 formed during these periods." 2 



In considering the time-ratios for the Paleozoic, Mesozoic, 

 and Cenozoic rocks of the North American continent, as given 

 by Dana and Williams, 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 limestone (from 10,000 to 

 20,000 feet) in the Cretaceous of Mexico. The limits of this 

 paper do not permit of 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 phenom- 

 ena accompanying their deposition, I would increase the com- 

 parative 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 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 Ceno- 



1 Journal of Geology, Chicago, Vol. I., 1893, pp. 294-295. 



2 Nature, Vol. 18, 1878, p. 268. 



