PROTECTIVE ICE CAP. 67 



the current, ^. e., along the lower half of the range where the auriferous 

 gravels are found. The waters of the glacial period started from the ends 

 of the glaciers at, say, 5,000 feet altitude with a very small load of finest 

 detritus. They were consequently in a condition to take up detritus or to 

 produce erosion at the same altitude at which the Tertiary streams were 

 overloaded and deposited gravel. 



The period which has elapsed in California since the glaciers disappeared 

 is a very brief one, and the canon erosion has no doubt been correspond- 

 ingly small. The precipitation is of course also small relatively to that of 

 the glacial period or of the Pliocene, and this explains the fact that in some 

 cases caiion erosion is still apparently progressing. It is known that a fall- 

 ing stream erodes the deposits which it has made at high water ; and it can 

 be shown by appeal to observation as well as by simple reasoning that a rel- 

 atively great rain-fall will tend to relatively great erosion of the upper part 

 of a drainage system, while, ceteris paribus, a small rain-fall will expend 

 most of its energy in eroding the lower part of the drainage. 



As a final argument against the tilting hypothesis I may mention that 

 there are preglacial gravels on the eastern slope of the range, e. g., in the 

 valley of the Truckee between Verdi and Carson. These gravels, like those 

 of the western slope, show terraces, and the stream has cut through them to 

 a depth of 100 or more feet. Tilting of the range as a rigid block 50 or 60 

 miles in width would of course diminish the eastern slope in the same pro- 

 portion that it would increase the western slope, and if erosion were inten- 

 sified on the latter it would be diminished on the former. 



On the whole, then, I cannot concede that any important tilting of this 

 portion of the Sierra has taken place at or since the post-Miocene disturb- 

 ances. 



Effects of the Fissures. 



Effect of irregular Distribution. — A few notes may next be made with ref- 

 erence to the effects of the fissure systems on the course of events in the part 

 of the Sierra here discussed. A^ has been mentioned, the distribution of 

 fissures is not uniform. In many places one system or the other is highly 

 and almost exclusively developed. Sometimes the rock is divided into very 

 regular prisms of indefinite length, and again these are cut by horizontal 

 partings into rectangular blocks. Finally there are areas in Avhicli the mass 

 consists of polyhedral fragments. 



The rate at which decomposition and erosion will take place clearly de- 

 pends upon the frequency of the fissures of a system and the number of 

 fissure systems developed in a given locality, for both erosion and disinte- 

 gration vary with the amount of surface exposed per unit volume. Thus, 

 where the granite is shattered into fragments of small size, disintegration 



