THE KOCKS OF THE WASHOE DISTRICT. 37 



the world. Such cases, liowever, demand very cautious treatment at the 

 hands of the geologist. Actual contacts are often exceedingly obscure, and 

 except where all the steps of a transition can be traced, such an explanation 

 of an anomalous occurrence is not justifiable. Fortunately nothing further 

 appears to depend either upon the specimen of diabasitic fragments inclosed 

 in a dioritic mass, or upon the diabasitic area in Ophir ravine, since the evi- 

 dence as to the succession of the rocks in the east countiy is decisive and 

 abundant. 



Other constituents. — Tlic feldspars are nearly or quite without exception tri- 

 clinic, and simple crystals are very rare, while pericline twinning is com- 

 mon. The stripes indicating polysynthetic structure are usually very 

 well defined, and of moderate width. The angles of extinction of a very 

 large number of favorably placed crystals have been noted, and seem to 

 indicate labradorite as the only feldspar present. Zonal structure is not 

 uncommon, but the feldspars are remarkably free from inclusions of any 

 kind, and are in general thoroughly transparent. 



Quartz is present in a large proportion of these rocks, though its dis- 

 tribution is very irregular, some slides containing only one or two grains, 

 while others show hundreds of them. Secondary quartz also occurs, but it 

 can usuallj' be distinguished from the primitive grains with ease. Primitive 

 quartz-grains are generally single, more or less imperfectly developed crys- 

 tals, around which grains of magnetite and other small cr3^stals are so 

 arranged as to show that their disposition has been controlled by the pres- 

 ence of the quartz. Secondary quartz occurs in veins or in patches composed 

 of granules of different cr3-stallographic orientation, and is not sharply 

 separated from the surrounding rock-mass. Secondary quartz, of course, 

 frequently carries fluid inclusions in rocks of all ages. The primitive 

 quartz of these diorites is rich in liquid inclusions, some of them vesicular 

 in shape, and others dihexahedral. The smaller ones show active bubbles, 

 and in some slides many contain salt-cubes. I have noticed none of the 

 appearances which accompany inclusions of carbonic acid, and in several 

 slides to which heat was applied no alteration in the size of the bubbles was 

 noticeable at a temperature considerably above 40° C. 



The iron ore is certainly for the most part magnetite, and I was unable 



