42 GEOLOGY OP THE COMSTOCK LODE. 



decomposed diorite and hornblende-andesite; and the only really safe coiirse 

 is to continne the examination nntil comparatively fresh specimens are 

 obtained. The grannlar structure of these is not readily confounded with 

 that of andesite. The diorites are never fissile like hornblende-andesite, and 

 hornblende-andesite is usually pretty uniform over considerable areas, while 

 the dioritic porphyries vary in structure almost from yard to yard. 



Mica-diorius. — The uiicaceous diorite-porphyries do not differ greatly from 

 the hornblendic variety, except in the substitution of biotite for hornblende; 

 but the rock is of a looser texture, tlie porphyritic feldspars are generally 

 larger, and tend more to rounded forms. I met with no occurrence of this 

 variety in a fresh condition. 



Relations of the diorites. — All vaHeties of dioHtc pa.ss ovcr into one another. 

 Porphyritic and micaceous forms occur in the prevailing granular mass tm the 

 front of Mount Davidson, directly opposite the Sarage mine, and granitoid 

 diorites occur, mixed with the porphyritic forms, on Cedar Hill and in the 

 McKihhcn Thidh'I. Especially in the latter locality gradations of the one form 

 into the other can be excellently followed out. The evidence of this character 

 is sufficient to jirove conclusively that no absolute separation can be estab- 

 lished between the dioritic rocks. Thei'e is, however, also considerable evi- 

 dence to show that, as a whole, one variety succeeded another in the course 

 of the eruption of the mass. The first jjortion of the diorites appears to 

 have been of the dark, fine-grained variety, and cases have been met with 

 in which dikes of the lighter rock cut the darker. In the mines, too, the 

 excavations show that the dark rock frequently underlies the lighter, and 

 in the deepest workings the dark predominates over the light rock. There 

 are not sufficient exposures of the coarse-grained diorites with brown 

 hornblendes to determine its relations to tlie varieties with fibrous horn- 

 blendes, but it evidently preceded the porphyritic rocks. The main mass 

 of the jiorphyritic diorite succeeded the granitic. Just to the south of 

 the Eldorado croi)pings there is a distinct dike of i)orphyritic diorite in 

 the granitic mass, with well-developed contact phenomena, extending about 

 an inch from each wall of the dike. To the north of Mount Davidson the 

 mine shafts, too, have gone down through porph3'ritic diorite into the granu- 

 lar variety. Indeed Mount Davidson, between Bullion ravine and Spanish 



