THE ROCKS OF TOE WASHOE DISTRICT. 39 



as wholly impracticable, charge after charge being shot from the drill-holos 

 as if they had been guns. Under the hammer it exhil)its no tendency to 

 break in one direction rather tlian in another, but weathering develops con- 

 siderable differences in resisting power; and in Bullion ravine, as maybe 

 seen from Plate VI., ridges and pinnacles have been formed by the irregu- 

 lar disintegration of the rock. Inmiediately west of the Lode the diorite is 

 furthermore divided into a system of approximately parallel sheets. It will 

 be seen in the next chnpter that I refer this system of Assuring to a fauhing 

 movement. 



Differ 



I from other rocks. — Thc gray granuLir diorite is uidikely to be con- 

 founded with any other rock in the district, except granular diabase. This 

 variety of diabase seldom occurs underground, so far as the country is now 

 open to inspection ; and when it is met with, as at the 3Imt connection in 

 the Sutro Tunnel, it is commonly limited to a very small body which shades 

 otf into finer grained varieties. When decomposition has progressed too 

 fiir to permit a macroscopical determination of the mineral constituents, the 

 lath-like development of the feldspars, the tendency to cleavage in parallel 

 planes, and a certain waxy luster will usually be found characteristic of the 

 diabase. 'I'lie dark fine-grained diorite has repeatedly been taken for ande- 

 site in the MeKihhcn Tunnel and elsewhere. The only resemblance, how- 

 ever, is in color, for the diorite shows to the naked eye a granular sti-ucture 

 never observed in the andesites of the District, although the latter are un- 

 commonly crystalline. 



porphyritic diorites. — Froui some peculiarity either in composition or texture, 

 the porphyritic hornblende-diorites have undergone very extensive decom- 

 position, and it was only after long and earnest search that two or three 

 small masses were found, which might furnish a study of this diorite in 

 a fresh state. A close inspection of fresh specimens shows that the rock 

 is even macroscopically thoroughly crystalline, but that tolerably well-devel- 

 oped feldspars and good hornblendes are separated out in a finer ground- 

 mass of a dark color. In addition to these minerals the microscope shows 

 magnetite, apatite, and zircon. Augite and mica also occur in limited areas. 



The hornblendes when fresh are bright brown and well crystallized, 

 often showing terminal faces as well as the prism and clinopinacoid. In a 



