Geol.— Vol. I.] TC'RiYER—OA'JGIA' OF YOSRMITE I' ALLEY. 283 



be rapid and heavy sliowers will move the blocks, wliile masses of large size 

 will remain unmoved and decompose slowly. The patches and zones where 

 the shattering has been relatively thorough will thus be carved into hollows 

 and ravines, while the more solid parts of the mass will remain as hills or 

 mountains. 



Forinatio>i of Canons. — The influence of these relations is very sensible in 

 the Sierra and every step of the process can be traced on a large or small 

 scale. The shattered zones commonly follow the direction of one of the 

 main fissure systems, and so do many of the canons, large and small; but 

 the zones sometimes jump across from one set of fissures to another parallel 

 set belonging to the same system, and so, too, do the canons. Such zones 

 also sometimes end abruptly — a fact due, no doubt, to variations in the com- 

 position of the rock. The canons do the same. 



Ice seems to have played a considerable part in clearing the cailons of 

 fragments and in excavating shattered and decomposed patches, so that in a 

 sense one must ascribe a large erosive eiTect to the glaciers; but the ice 

 seems, nevertheless, to have been incapable of cutting into solid masses to 

 any extent, or even into much fissured rock where little decomposition had 

 preceded and where the blocks were tightly wedged together. * * * 



In the area here dealt with there are gorges resembling the Yosemite Val- 

 ley in the most striking manner, though on a small scale, but so exposed as 

 to show that their existence is due simply to local intensification of the shat- 

 tering process. In one especially striking instance in the district known as 

 Border Ruffian the gorge is on the east-northeast vertical fissure system, but 

 the fissures are slightly inclined to the southward through some irregularity 

 in resistance. Faulting has consequently produced irregular cross-fractures 

 of the mass and reduced it to more than ordinarily small fragments. Weath- 

 ering, water and ice have then done their perfect work and cut the mass 

 down to the lowest possible level, leaving, however, the rocky floor exposed. 



* * * If the fault system studied in this paper extends to the Yosemite, 

 as the fissure system certainly does, the valley cannot be due to a local sub- 

 sidence. On the other hand, I can see no objection to the hypothesis that 

 the shattered zone of rock was disintegrated and eroded in pre-glacial times, 

 the process being completed by a glacier which left a small moraine near the 

 entrance and thus converted the valley into a lake. 



Whitney,^ Gilbert," Russell,^ and Becker* agree in suppos- 

 ing the present drainage system of the Sierra to have been 

 far advanced before the time when the ice occupied the 

 canyons, while Muir believes that ice carved out all the 

 canyons of the Sierra Nevada. It is, therefore, to the point 

 to examine into the question of the power of ice to erode. 



1 Whitney, Geol. Cal., Vol. I, 1865, p. 421, and elsewhere. 



2 Gilbert, Mon. I, U. S. Geol. Surv. i8go, p. 269. 



* Russell, Eighth Annual Report, U. S. Geol. Surv., Part I, 1SS9, p. 349. 



* Becker, Bull. Geol. Soc. Amer., Vol. II, 1891, pp. 67-69. 



