48 GEOLOGY OF THE COMSTOCK LODE. 



niiiio, iiiul tliiit tlic (luurtzes in each of tliem contained fluid inclusions with 

 moving bubbles. The one slide which Professor Zirkel determined as rhy- 

 olite difl'ers, in that the quartz contains glass but no fluid inclusions; in a slide 

 ot" my own, however, from as nearly as possible the same locality, these con- 

 ditions are reverscil, the quartzes showing fluid inclusions but none of glass. 

 I can see no ditTerence in the amount of orthoclase present in those slides 

 determined respectively as dacitt' and rhyolite. Professor Zirkel gives an 

 analysis of this i-ock made b}- Mr. Councler, showing two per cent, of soda 

 and three and six-tenths per cent, of potash. In discussing this conq)osition 

 Professor Zirkel cites a mnnber of analyses of Transjdvania dacites, but in 

 none of these is the proportion of potash to soda so high as in the Washok 

 rock. 



DIABASE. 



Earlier diabase. — There are two Varieties of diabase in the district. The 

 older of these forms the lianging wall of the Lodk; the other has been 

 known as "black dike." The east-country diabase varies considerably in 

 coarseness of grain and in color. Wlien really fresh it is always dark, and 

 w hen also fine-grained it closely resembles an andesite. The coarser-grained 

 and somewhat decomposed occuirrences are often confusingly like granitoid 

 diorite. 



The rock consists of j)lagioclase, augite, and an iron ore, with a num- 

 ber of accessory and irregularly distributed minerals, quartz, hornblende, 

 mica, and apatite. The structure is not that most usually found in diabases, 

 being somewhat })orphyritic. The augite is of the usual pale-brown tint, 

 and occurs largely in well-developed crystals. These are often twinned 

 according to the ordinary la.w.^ The twinning attracts more attention than 

 usual, because polysynthetic structure is common, some of the lamelhii often 

 penetrating only part way through the crystal. The ordinary cleavages are 

 well marked, and instances are common in which the pinacoidal cleavages 

 as well as the prismatic ones are developed. Some slides contain only sepa- 

 rate and well-formed crystals, while in others they occur in groups, and 

 these are apt to be gathered about branching masses of iron ore, almost like 



'For a peculiar case, whicli might be interpreted as abaormal, see page 113. 



