GEOLOGY OF THE EUREKA DISTRICT. 



confirmed and extended by an examination of the lavas from the volcanoes 

 of the Pacific Coast' and those of the Great Basin. 2 In the investigations of 

 these latter, the pyroxene-andesites of Richmond Mountain played an in- 

 teresting part, all the more important as a large suite of rocks from one 

 locality whose field relations were known were subjected to a most careful 

 mineralogies! study. The isolation of the hypersthene from augite was 

 accomplished by means of a solution of cadmium-boro-tungstate having a 

 specific gravity of 3 '3 9. Repeated treatment with the solution yielded a 

 brown hyperstheue carrying a small amount of green augite. Under the 

 microscope the former proved to be orthorhombic in form and strongly 

 pleochroic, the latter monoclinic and without pleochroism. In the greater 

 part of the pyroxene-andesites of Richmond Mountain the hypersthene was 

 always found to be in excess of the augite, the prevailing minerals being 

 hypersthene and anorthite, accompanied by labradorite and possibly other 

 plagioclase feldspars, together with varying amounts of augite and 

 magnetite. 



Basalts. Under basalts are included those volcanic rocks which have for 

 their essential ingredients plagioclase augite and magnetite. Olivine, which 

 occurs as a common accompaniment in varying proportions, in many vari- 

 eties, is, however, too frequently wanting to be rigidly regarded as an 

 essential constituent. The basalts form the most basic of all natural groups 

 into which the volcanic lavas have been divided. At Eureka they present, 

 for the wide field which they cover and the great number of their extru- 

 sions, a uniform appearance, and, although characterized by a large amount 

 of glass base, may be regarded as typical of many localities in Nevada. 

 It is fine grained and compact, frequently passing into vesicular forms, with 

 but few macroscopic secretions, and by far the greater part of it grayish 

 black in color. Olivine occurs in large grains and in such quantities as 

 occasionally to modify the external character of the rock ; yet over broad 

 areas it is wholly wanting, the microscope failing to detect its presence in 



1 Notes on the Volcanoes of Northern California, Oregon, and Washington Territory. Hague 

 and Iddings. Am. Jour. Sci., September, 1883, vol. 26, pp. 222-235. 



'Notes on the Volcanic Bocks of the Great Basin. Hague and Iddiugs. Am. Jour. Sci., 

 Jane, 1884, vol. 27, pp. 453-463. 



