FELDSPAR DETERMINATIONS. 407 



not prove that none is present in the rock, but only that it is comparatively infrequent. 

 That it was not the predominant feldspar was also inferred from the micro8coi)ic 

 examination. 



Augite-andesite, peak south of Crown Point Eavine, marked 7075. 



Anorthite, bytownite, and labradorite were detected in this specimen. The 

 anortliite was found under the microscope, and its presence prevented the detection of 

 labradorite, except among the microlites. But on page 64 it is stated of the augite- 

 andesite that, though " anorthite has been identified in a few slides, • • * in most 

 cases the maximum angles of extinction correspond to labradorite." 



Augite-andesite, above Ophir grade, due west of Belcher hoisting-works. 



This, too, showed anorthite, labradorite, and an intermediate variety. Anorthite 

 was found also in the augite-andesite from Basalt Hill. 



Later hornbleude-andesite, quarry 2,000 feet northeast of Sutro Tunnel Shaft III. 



All of the feldspars tested (more than a dozen) gave very sharp reactions for 

 andesine.i Nearly all of them show zonal structure. 



Later hornblende-andesite, quarry 2,000 feet east of Occidental Mill. 



An andesine, an oligoclase, and several crystals of intermediate composition 

 were found. This accords excellently with the analyses of these feldspars made by 

 Mr. Dewey .^ 



Later hornblende-andesite, quarry above Utah mine. 



Labradorite, andesine, and an intermediate variety were detected. 



It was not found practicable to examine the feldspars of the later diabase (black 

 dike) or the basalt, on account of the fine grain of these rocks. 



On the whole, the examination strongly confirms the results of the microscopical 

 analysis, and the only rocks in which the flame-reactions revealed feldspars which 

 might have been detected by the microscopic method are the granite and the quartz- 

 porphyry, each of which shows, in addition to oligoclase, an unsuspected, more basic 

 feldspar, which is, however, more closely allied to oligoclase than to labradorite. 



While the optical behavior of a few of the large feldspars in the Washoe 

 andesites indicates that they are mixtures of different species, this is exceptional. 

 They are ordinarily polysyuthetic individuals, showing only two angles of extinction. 

 In a large proportion of cases the crystallographic relations of the twinned lamellfe 

 are further emphasized by the presence of zonal structure. Granting the accuracy of 

 Szabo's method, it is therefore extremely difiBcult to suppose that the prevalence of 

 crystals giving the flame-reactions for andesine in the andesites, and particularly in 

 the rock from the quarry northeast of Sutro Shaft III., is due to aggregations of two 

 distinct species. The uniformity of the reactions obtained is also an argument against 



' Professor Szab6 examined two crystals from this specimen, and pronounced them very charac- 

 teristic andesine. 



' See pages 67 .and l,'i4. 



