PEOPYLITE. 87 



ular as to be nearly indistinguishable from Mount Davidson rock, while 

 others are dark fine-grained porphyries closely resembling andesites. The 

 latter, however, can be shown from slides to be dioritic, while the granular 

 varieties are as like the Mount Davidson rock microscopically as they 

 appear to the naked eye. A portion of these rocks is altered, but the transi- 

 tions from the fresh to the decomposed state can be studied more satisfactorily 

 in the McKibhen Tunnel, because, in the ravine, decomposition is most preva- 

 lent in the bluifs near the andesite, whicli is also somewhat altered. There 

 is no evidence of any contact between these bluffs and the unquestionable 

 diox'itic masses adjoining them, and in spots where the rock is comparatively 

 fresh, its character seems unmistakably the same; but when the effect of 

 decomposition on the tunnel porphyries is considered in reference to the 

 ravine rocks, it becomes clear that the blufl's can be only altered forms of 

 the adjacent varieties of diorite. 



Crown Point Ravine. — Ouc flank of Crowu Poiut Raviuc shows tolerably fresh 

 hornblende-andesites, the other excellent fresh augite-andesite. Near the 

 drainage the rock is largely a highly decomposed breccia, in part bleached 

 to whiteness, but the area occupied by propylitic rocks is very small and 

 could only represent an exposure by erosion. As is common in breccias, 

 the decomposition is not uniform. The matrix is so altered that its coher- 

 ence is a matter of surprise, and many of the included fragments are tinged 

 with epidote. Some, however, from superior -density or accidental protec- 

 tion are less aflfected, and a few large uufissured blocks are tolerably fresh 

 at some distance within their surfaces. Wherever the inclosed masses are 

 fairly fresh they look like andesites, and under the microscope there proves 

 to be no distinction, when the course of chloritic decomposition known 

 from other occurrences is allowed for. 



South Twin Peait ("Gold Hill Peak"). — The South Twiu Pcak looks moi'c like the 

 younger gray hornblende-andesite of the Utah quarry than the more usual 

 varieties of the earlier eruption, while the northern peak is for the most part 

 normal; but it also shows occasional small patches resembling its southern 

 neighbor, and there seems a gradual transition from one to the other. Fresher 

 specimens from the South Peak show abundant lustrous, seemingly black, 

 hornblendes with perfect cleavage, and these under the microscope prove to 



