INTRODUCTION 



63 



the amount of helium in a sample rock and the amount produced in the 

 sample in one year, we can reckon the length of time the helium has been 

 accumulating, and hence the age of tlie rock. This method maj' l(>ad to 

 determinations not merely of the average age of the crust of the rock, but 

 of the ag(\s of particular rocks and tlu; date at which the various strata were 

 deposited.^ 



A very rough estimate of the accumulation or thickness of the Eocene 

 and Oligocene sediments in the Rocky Mountain basins is seven thousand 

 feet. This is an approximate figure which will in time be made exact. 

 These sediments, however, consist largely of tufTs or partly worked over 

 volcanic materials deposited in water. We certainly have no means of 

 comparison with similar processes going on to-day which will enalile us to 

 estimate the time occupied in the accumulation of these rocks. 



Thus difficulties confront us on every side, and the most careful of our 

 computations are mere approximations. Since, however, it is desirable 

 to give some idea of the scientific opinion on the duration of the Age of 

 Mammals, the following table is of interest. 



Length of C^nozoic Era or Age of Mammals 



Estimated by Comparison with Present Rates of Deposition and Denudation 



Dana 1874 



WaUace 1881 



Walcott 1893 



Upham 1893 



3,000,000 years 



4,200,000 years 



(Tertiary = 

 4,000,000 



Quaternary = 

 200,000+) 



2,900,000 years 



3,100,000 years 

 (mean) 



(Tertiary = 

 2 - 4,000,000 



Quaternary = 

 100,000) 



Based on the estimated thickness of the 

 total series of stratified rocks and the 

 estimated rate of accumulation of de- 

 posits along the shores of continents 

 at the present time. 



Based on the rate of denudation with the 

 estimated thickness of sedimentary 

 rocks (which is probably less than 

 177,200 feet, as given by Haughton 

 '78); and further on the dates of 

 phases of high eccentricity of the 

 earth's orbit. 



Based on the total thickness of sedi- 

 mentary rocks of North America 

 (100,000 feet), compared with i)resent 

 rates of accumulation. 

 Based on estimates of the length of the 

 glacial stages. 



* Thomson, .7. J., Address of the President of the British Association for the Advancement 

 of Science [Winnipeg, 1909J. Science, n.s. Vol. XXX, no. 765, Aug. 27, 1909, pp. 257-279. 



