380 GEOLOGY OF THE OOMSTOCK LODE. 



except those near enough to the main cropping to reach the surface where 

 it has a very considerable slope, are likely to be continuous. 



Finally, it is shown that the law of land slips is also capable of expres- 

 sion by logarithmic equations, and that a large part of the details of the 

 topography of grassy hills is formed in obedience to this law. 



OCCURRENCE AND SUCCESSION OF ROOKS. 



Succession. — Tlic succcssiou of rocks made out in the Washoe District is 

 as follows: Granite, metamorphics, g-raiiular diorites, porphyritic diorites, 

 metamorphic diorites, quartz-porphyry, earlier diabase, later diabase ("black 

 dike"), earlier hornblende-andesite, augite-andesite, later hornblende-ande- 

 site, and basalt. 



Granite and metamorphics. — Granite occurs ou tlic sui'face ouly in a very lim- 

 ited area near the Bed Jacket mine, but it is certain that it has a considerable 

 underground development, for it has been struck at the Baltimore, the Bock 

 Island, and b}" a tunnel to the southwest of the latter beyond the limits of 

 the map. 



The granite is overlaid by metamorphic rocks, which, however, are 

 less metamorphosed close to it than at a distance from it, and the probabil- 

 ities are that the sedimentary strata were laid down upon the massive rock. 

 The sedimentaiy rocks are limestones, crystalline schists, and slate. They 

 are badly broken and highly altered, and the search for fossils was not 

 rewarded by success; but the general geology of this part of the Great 

 Basin leaves little doubt that they are Mesozoic. A considerable area 

 of metamorphics has been exposed in the southwest of the region by the 

 erosion of the overlying eruptive masses. North and east of Silver City, 

 however, the surface shows scarcely any metamorphics, while they play a 

 large part in the underground occurrences as far as the Yellow Jacket. In 

 the Gold Hill mines black slates form the foot wall of the Lode. They are 

 intensely colored with graphite, and often very highly charged with pyrite. 

 They are frequently mistaken for '.'black dike," but a moment's inspection 

 in a good light shows their sedimentary origin. The presence of such car- 

 bonaceous rocks at greater depths would explain the formation of hydrogen 



