92 GEOLOGY or THE COMSTOCK LODE. 



magnetite, and some accessory minerals. The structure is typically granit- 

 oid, none of the principal minerals showing either perfect crystalline out- 

 lines or microlitic development. The orthoclase is for the most part trans- 

 parent, and in many cases shows good cleavages, which ai-e usually parallel to 

 the extinctions. The plagioclases show very narrow stripes and no angles of 

 extinction exceeding those of oligoclase. The quartz contains abundant liquid 

 inclusions, many with moving bubbles. The mica shows the interference 

 figure of biotite, and is of course brown and highly dichroitic. A portion 

 of the biotite appears "bleached" to a lighter brown, and other fragments 

 are converted into chlorite. A few particles of epidote are visible, forming 

 from the chlorite. The iron ore is evidently magnetite, occurring mostly in 

 quadrangular forms, and being accompanied by hematite. There is also a 

 considerable amount of titanite, which in some cases takes the form of per- 

 fect rhombs, with an angle of somewhat less than 140°. It shows the 

 cleavages, the rough surface, high refraction, and dull colors between crossed 

 Nicols, appropriate to sphene. There are many minute zircons, and some 

 ordinary apatites. The slide contains two patches of a somewhat highly 

 refracting, nearly colorless, slightly yellowish, mineral, one of which seems 

 to be of an imperfect hexagonal outline, and the other nearly square. They 

 show a rippled surface, such as is often seen on augite. They remain 

 dark between crossed Nicols, and give no interference figure. The mineral 

 shows cracks, some of which are irregular; others seem referable to an im- 

 perfect rhombohedral cleavage All these properties suggest sodalite. This 

 mineral, however, has been noticed, I believe, among the older massive 

 rocks only in syenite,^ and in combination with elaeolite and zircon. As 

 zircon is plentiful in this slide, I carefully looked for elaeolite. If present 

 at all it must be in granitoid crystals, which might be mistaken for ortho- 

 clase. Many such are cut nearly at right angles to an optical axis, but I 

 failed to find one such which gave the interference figure of a uniaxial min- 

 eral. 



'As the name ie now usnally nndeistood. In Dana's Mineralogy the quartzless mica-orthoclase 

 rocks are still termed granite. 



