364 GEOLOGY OF THE EUREKA DISTRICT. 



that the rocks forming Richmoud Mountain and Cliff Hills are pyroxene-andesites, 

 with a very considerable percentage of hornblende as an essential ingredient and 

 biotite as an accessory one. They might in fact be termed hornblende-pyroxene- 

 andesites. 



A very striking correspondence between the different varieties in each of the 

 two pyroxene-andesite occuijreuces in the district will be seen on comparing together 

 thin sections 102, 104 with 77, 78, 79; 107 with 90, and 108 with 92. The other sec- 

 tions from Richmond Mountain have corresponding varieties at Cliff Hills, which, 

 however, were not made into thin sections. Several thin sections remain, which n(!('d 

 a brief mention. No. 109 is from a modification of the Cliff' Hills rock, which in s(mie 

 respects resembles the basalt of jMagpie Hill and that on the soutli slope of Alhambra 

 Ridge, but which is found under the microscope to be a much finer grained pyroxene- 

 andesite, rich in magnetite, with phenocrysts of the same feldspar and pyroxene, but 

 without hornblende or mica, and bearing some small red altered crystals of olivine, 

 whose presence might throw considerable doubt over the determination were there 

 not frequent patches, not inclusions, of coarser grained groundmass free from magnet- 

 ite, identical with the groundmass of the neighljoring andesite. Thin section 110 is 

 a variety from a limited exposure in the tuff' northwest of Devils Gate, which is poor 

 in large phenocrysts. 



Hornbiende-[Mica]-Andesite.— The hornblende-[mica]-andesite of the district, though 

 closely related to the pyroxene-andesite in many respects, has sufficient strongly 

 marked points of contrast to constitute a separate rock. The areas of exposure of 

 the two in the field are nowhere in contact, and no transition of one into the other is 

 detected under the microscope, except in the andesitic pearlites to be described later. 

 The hornblende- [micaj-andcoite has a light puride and reddish purple groundmass rich 

 in macroscopic crystals of feldspar, hornblende, and biotite, of which the feldspar i)re- 

 dominates ; it is further characterized by the total absence of pyroxene. Several mod- 

 ifications which occur in separate and limited exposures will be noticed in their proper 

 connection. Under the microscope the rock (thin sections 38, 41, 35, 37, 42, 42a, 39) 

 is seen to consist of a microcrystalline feldspathic groundmass, in some cases entirely 

 free from glass base. It is rich in macroscopic phenocrysts of feldsi)ar, black bordered 

 hornblende, and reddish brown biotite. The accessory minerals are apatite and very 

 little magnetite, and zircon; quartz is an accessory mineral in some occurrences, espe- 

 cially in that east of Pinto Road (39). 



The feldsjiar is wholly triclinic, being for the most part striated, and the 

 unstriated sections giving angles of extinction belonging only to plagioclase. The 

 porphyritical crystals are beautifully developed, yielding sharply outlined sections, 

 one or two millimeters in length, of the usual form. They show remarkably fine zonal 

 structure, well illustrated in Fig. 1, PI. v. In this feldspar, besides the successive 

 stages when the crystal liatl rectilinear outlines, there were three periods when its 



