62 THE AGE OF MAMMALS 



side of the River Nile. It is found that by the annual overflow the 

 sediments accumulate at the rate of two feet and ten inches in 100 years.^ 

 Again, illustrating the difficulty of forming estimates from present rates of 

 accumulation or deposition, the estimates given by Geikie in 1892 may be 

 citetl, which show that the sedimentary deposits at the mouth of the Po 

 are much more rapid than those at the mouth of the Danube. 



Denudation or erosion. — Estimates based on denudation confront 

 similar difficulties. Haughton - in 1878 found the mean rate of denuda- 

 tion of the surface in the several great river basins of the world to be one 

 foot in 3090 years. A most ingenious method of measuring the rate of 

 erosion is the ' cedar-root chronology, ' which appears to have been invented 

 by James Hall ^ in 1871. He made an elaborate study of the rate of ero- 

 sion along the valley of the Mohawk River in New York, based upon the 

 estimated age of nineteen cedar trees, the length of the exposed root, and 

 the recession of the cliff per century. From this he calculated that 35,000 

 years was the minimum of time since the Cohoes Falls were opposite the 

 pothole in which the famous 'Cohoes mastodon' skeleton was deposited. 

 The same method was used by Knight ^ in 1899, of especial interest be- 

 cause the observations were made in a well-known fossiliferous area, at 

 Bates-Hole, Wyoming, where there is a vast depression produced by the 

 erosion of the Tertiary beds of Oligocene Age. On its slopes grow pine trees 

 (Pinus murrayana Eng.) that have recorded the rate of erosion here for 

 about 300 years. As the material was worn away their roots became 

 more and more exposed. The oldest of the trees stand on slopes, their 

 trunks elevated three or four feet above the slopes. On the average it was 

 found that the trees 300 years old had about three feet of rock removed 

 from their roots. According to this 100 years are required to remove one 

 foot of surface. Three miles have been eroded on either side, and at the 

 rate of one foot per century, 1,584,000 years must have elapsed since the 

 process began. The process began not earlier than the close of the Miocene, 

 when the highest beds of Bates-Hole were deposited. Thus the erosion 

 must have occurred during the subsequent Pliocene and Pleistocene periods, 

 which estimated in this way represent a duration of 1,584,000 years. On 

 this basis it would not be out of the way to estimate the Age of Mammals 

 at 4,000,000. 



Helium content. — The most recent method is that of Strutt, based 

 upon the amount of helium found in different rocks. Helium, like the 

 radioactive elements, accumulates in minerals, and hence if we measure 



1 Lyons, H. G., The Physiography of the River Nile and its Basin. Surv. Dep't. Egypt, 

 Cairo, 1906. 



- Haughton, Physical Geology. Nature, Vol. 18, 1878, pp. 266-268. 



^ Hall, J., Notes and Observations on the Cohoes Mastodon. Rept. N'. Y. State Cab. Nat, 

 Hist., Vol. 21, 1871, pp. 99-148. 



* Knight, W. C, Some New Data for Converting Geological Time into Years. Science^ 

 n.s. Vol. X, 1899, pp. 607-608. 



