356 



GEOLOGY OF THE EUltEKA DISTKICT. 



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

 instances an uncompleted black border has been added to the primary augite, in every 

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

 in, 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 ocP 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 orthopiuacoid 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 parallel to their cleav- 

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

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

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

 to be augite. But the observations of Cross 1 on the hypersthene-andesites of Colo- 

 rado and oth.er localities, and our own observations on the andesites of the volcanoes 

 of northern California, Oregon, and Washington Territory, 2 and on the volcanic rocks 

 of the Great Basin, 3 and the studies of many other observers, in different parts of the 

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

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

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

 cadmiumborotuagstate solution, as already described iu the paper on the volcanic 

 rocks of the Great Basin just mentioned. The pyroxene was found to consist of green 

 augite and brown hypersthene; 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 hypersthene and augite was calculated, resulting as follows : 



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



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



3 Am. Jour. Sci., June, 1884, vol. XXVH, 



