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University of California Publications. [Geology 



have been produced on the jointed granites at Silver Lake, and 

 it is obviously apparent that the phenomenon is independent of 

 lithologic control. 



That the term "quarrying" is no mere figure of speech is 

 forcibly brought home by the photograph shown in Plate 29B. 

 This was taken on Crystal Creek at an altitude of 10,000 feet 

 in a valley hung up 2,000 feet above Mineral King. No less 

 than five benches are shown, scarcely distinguishable in their 

 perfection from the labored handiwork of man. They are 

 quarried out of a strongly sheared andesite in steps up to fifteen 

 feet high, and display the efficient utilization of the two dominant 

 jointages. The ineffectiveness of glacial abrasion compared to 

 the process of plucking is also clearly revealed. 



It has been mentioned that on Monarch Creek along with the 

 numerous quarry-like benches evidences of a glacially striated 

 notch occur in the bottom of the trough. Similar subglacial 

 channels are reported from the Unter Grindelwald glacier and 

 from the Yosemite Valley, and are held to furnish proof of the 

 feebleness of glacial erosion. At Monarch Creek, however, the 

 evidence is clear that the subglacial notch was an active co- 

 operant with the glacial processes. The heavy grade of the 

 stream limited its energies to the downward corrasion of its 

 bed, and caused the incision on the floor of the valley of a narrow 

 channel. To the ice was left the quantitatively much greater 

 task of excavating the main trough and maintaining the typical 

 character of the U-profile. It is a well-known principle to the 

 quarryman that the facility with which large blocks can be re- 

 moved is greatly increased by the number of free faces exposed 

 to attack. The downward channeling action of the subglacial 

 stream, therefore, afforded a vantage line for the quarrying 

 ability of the ice, and has enabled it, by utilizing the latent free 

 faces — the jointage planes, — to evolve the characteristic bench- 

 like forms. 



THE GRANITIC AND ASSOCIATED IGNEOUS ROCKS. 



The Granitic Rocks. — The granitic rocks of the region com- 

 prise several strongly marked types, and represent somewhat 



