356 



GEOLOGY OF THE EUliEKA DlSTlilCT. 



presence is not uuiversiil, some pyroxenes being entirely free from it. In a very few 

 instances an uncompleted black border lias been added to the primary angite, in every 

 case projecting beyond the crystal outline of the remainder of the surface, Fig. 3, PI. 

 Ill, and being inclosed in the narrow margin just described. This black border appears 

 to be an aggregation of magnetite grains. A zonal structure is occasionally noticed. 

 The prismatic cleavage parallel to aP is quite perfect in some crystals, but in others 

 it is nearly lost in irregular fractures. The crystals are mostly simple individuals; a 

 few are twinned parallel to the orthopinacoid and show three or four alternating bands 

 between crossed nicols. 



At the time when these rock sections were studied it was considered probable 

 that all the pyroxene individuals observed in any one rock belonged to the same 

 species, and that those sections with the axes of elasticity ijarallel to their cleav- 

 age or to the trace of the faces in the prism zone were sections cut in the zone at right 

 angles to the cliiiopiuacoid of augite, when they were accompanied by other sections 

 with inclined position for these axes. Hence all the iiyroxene in this case was thought 

 to be augite. But the observations of Cross' on the hypersthcue-andesites of Colo- 

 rado and other localities, and our own observations on the andesites of the volcanoes 

 of northern California, Oregon, and Washington Territory,' and on the volcanic rocks 

 of the Great Basm,^ and the studies of many other observers, in dift'erent parts of the 

 world have demonstrated the joint occurrence of an orthorhombic and a monoclinic 

 pyroxene in a great variety of rocks. Moreover, the ]3yroxeue of this particular ande- 

 site from Eichmond Mountain has been separated from the rock by means of the 

 cadmiuraborotuugstate solution, as already described in the paper on the volcanic 

 rocks of the Great Basin just mentioned. The i>yroxene was found to consist of green 

 augite and brown hyperstheue; the latter was isolated with a small admixture of the 

 augite and analyzed. From the composition of the whole, analysis I, a theoretical 

 composition for the hyj)ersthene and augite was calculated, resulting as follows : 



II. 

 Hypers- 

 thene. 



III. 



Aueite. 



SiO.2 

 AUOa 

 TiOj 

 FeO. 

 MnO 

 MgO 

 CaO. 

 Igu. 



51-16 



3-50 



•73 



15-46 



-56 



19-22 



8-84 



•42 



99-89 



51-39 



3-26 



•73 



16-45 



-56 



19-75 



7-31 



•42 



99-87 



49^02 



5-64 



•73 



6^45 



•56 



14^37 



22-60 



-42 



99-79 



' Am. Jour. Sci., 1883, vol. xxv, pp. 139-144. 

 'Am. Jour. Sci., Sept., 1883, vol. xxvi. 

 'Am. Jour. Sci., June, 1884, vol. xxvil. 



