GRANITE. 143 



showed considerable quantities of soda in all excepting the last. Quan- 

 titative determinations of the alkalis in ((3) were then made, giving 3.08 

 per cent, potash and 5.54 per cent. soda. Thi.s, however, does not settle 

 the matter, since it is well known that orthoclases sometimes contain 

 more potash than soda.' The specific gravities of these minerals also 

 vary greatly, but it is certain that many specific-gravity determinations 

 have been made with impure material. No less an authorit}^ than Professor 

 Tschermak gives the specific gravity of orthoclase as 2.558; albite, 2 624; 

 anorthite, 2.758. These figures, on Tschermak's theory, would give oligo- 

 clase at 2.658. If the specific gravities of (5), (6), and (7) be assumed to 

 be each 0.01 higher than that at which they fell and if the mixture of 

 orthoclase and oligoclase, which would give these specific gravities, be com- 

 puted at Professor Tscliermak's figures, it will appear that the granite in 

 question must have contained almost exactly equal quantities of oligoclase 

 and orthoclase. Taking into account its association with less plagioclastic 

 rocks, its habitus, etc., there can be no doubt that it is to be classed as a 

 granite, and not as a plagloclase rock. 



There are light-colored bands in the granite at Steamboat Springs 

 which bear the appearance of dikes of granitic rock. Under the mi- 

 croscope these dike-like masses are found to be somewhat decomposed 

 granite-porphyry, showing rounded grains of quartz, orthoclase, oligoclase, 

 and remains of ferromagnesian sdicates in a microcrystalline groundmass 

 which appears to consist mainly of orthoclase and quartz. The larger 

 quartzes in this rock show abundant fluid inclusions. These dikes were 

 not observed to penetrate the overlying metamorphic rocks, and are 

 probably older than the latter, if not substantially of the same age as the 

 granite. 



The granites collected from the eastern ridges of the Sierra are not 

 distinguishable from those of Steamboat Springs ; but a granite from 

 Washoe Lake shows exceptionally well developed, long hornblende-crystals 

 embedded in a very white quartz-feldspar hiass. The mica is less promi- 

 nent. 



' Compare Daua: Syst. of Mill. ; and Rolh: AIljj. uud cUem. Geo!. 



