SUMMARY. 381 



sulphide. There is also an obscure occurrence of metamorphic limestones 

 in the Sierra Nevada mine between granular and micaceous diorite. It 

 appears to be conformable to the face of the granular diorite. The meta- 

 morphics in and about Gold Hill seem both to overlie and to underlie 

 diorite, and there is little doubt that sedimentary strata were present at the 

 period of the diorite eruption. 



Between the metamorphics and the quartz-porphyry in the southwest 

 portion of the area is a considerable extent of metamorphic diorite. In 

 some occurrences this rock is a distinct breccia, and bears a strong resem- 

 blance to augite-andesites or basalts, while elsewhere it is extremely like 

 Mount Davidson diorite. Besides the surface occurrences, it is found par- 

 ticularly well developed in the Silver Hill mine. 



Diorites. — Tlic principal exposure of diorites is on the west of the Lode 

 through Virginia City, but there are several outlying occurrences about 

 the Forman shaft, and again far to the east at the Lady Bryan mine, which 

 show that the underground development of the rock is a very extensive one. 

 It forms the foot wall of the Lode from the Yellow Jacket north. The diorite 

 is excessively uneven in its composition, and in almost any area of a hun- 

 dred feet square several modifications are to be found. This fact, taken in 

 connection with the microstructure of the rock, is pretty conclusive evidence 

 that it has never reached a higher degree of fluidity than the plastic state. 

 The varieties can be roughly classified as granular diorite, porphyritic horn- 

 blendic diorite, and porphyritic micaceous diorite. But intermediate varie- 

 ties are of constant occurrence. There seems, nevertheless, to be a certain 

 amount of order in the disposition of the difterent varieties. Mount David- 

 son, from Bullion Ravine to Spanish Ravine, is almost altogether granular, 

 but to the north and south of these hmits porphyritic forms prevail. In 

 the neighborhood of the Utah mine mica becomes the predominant ferro- 

 magnesian silicate, and this variety is also the one which occurs in the 

 neighborhood of the Forman shaft. How this orderly disposition of the 

 various diorites came about is a somewhat obscure question, as a possible 

 answer to which an hypothesis is advanced. 



Diabases. — The diabase appears but to a very trifling extent upon tLe 

 surface, though it is by no means unlikely that an exposure of this rock 



