SOME ASPECTS OF GEOLOGIC TIME 251 



The structure of the crust of the earth contains within itself 

 so many signs of the manner of its formation, that it is surely 

 possible to disentangle valid methods, if only the geologist will 

 diligently search them out. If he will cease from following 

 false clues, it is not impossible that he may, even now, be on 

 the way to clearer and more certain knowledge. 



Towards the accomplishment of this end, it is, as yet, 

 impossible for any single worker to do more than to make a 

 few tentative suggestions. As the question is seriously attacked, 

 and as it is made the subject of careful and detailed research, 

 new paths will open, and new methods will be discovered. 

 Meanwhile, it will be of interest to note a number of possibilities, 

 the full bearing of which the geologist of to-day is liable to 

 overlook. 



Let us first consider the use that can be made of the data we 

 are supposed to possess concerning the rate of erosion. The 

 discharge of sediment at the mouths of a number of rivers has 

 been measured, and, by these measurements, geologists have 

 attempted to estimate the rate at which the continents are being 

 carried to the ocean. But difficulties arise when we attempt to 

 obtain from our data a general average rate of denudation, 

 especially such as it is possible to apply to previous geologic 

 epochs. The rate of erosion must vary enormously. In a 

 rainless district, such as the canons of the Colorado, it is very 

 slow. In a country of torrential rainfall, such as the Ganges 

 basin, it is very great. The question, therefore, must be faced 

 which conditions can be regarded as typical. The rivers men- 

 tioned by Geikie, concerning which reliable measurements 

 exist, are the Mississippi, the Ganges, the Hoang-Ho, the Rhone, 

 the Danube, and the Po. To these Chamberlin adds the 

 Potomac, the Rio Grande, the Uraguay, the Nile, and the 

 Irrawady. 1 The majority of the data measure the transport of 

 alluvium from irrigated and cultivated soils. Small particles 

 of alluvium are carried a short distance, and are either deposited 

 elsewhere in the basin or in the region of the slowly forming 

 delta. To interpret correctly what this transportation means 

 requires careful thought and analysis. The discharge may 

 represent the normal and average lowering of the level of the 

 river basin. But there is another possibility which must not be 



1 See Geikie, Geology ■, p. 589 ; Chamberlin and Salisbury, Geology ', etc., vol. i. 

 p. 101. 



