OLIVINE IN HAS ALT. 257 



their structure a uniform grotmdmaaa made up of coarse-grained aggrega- 

 tions of feldspar and augite, imbedded in a globulitic glass base. 



Nearly all the rocks of intermediate mineral composition possess the 

 basaltic habit. Hypersthene is wanting in the normal basalts. Augite and 

 magnetite, although essential minerals in the composition of both rocks, occur 

 much more abundantly in basalts. With one exception the microscope has 

 failed to detect olivine in any thin section of the lavas classed as pyroxeue- 

 audesite, the exception, however, furnishing quite a remarkable rock, and 

 one that might with some reason be placed among the basalts. It occurs 

 in an obscure exposure or knoll in Fish Creek Valley just west of Cliff 

 Hills, and from its association, and still more from the fact that its ground- 

 mass structure bears the closest relation to adjoining rocks, it has been 

 referred to the pyroxene-andesites. Although olivine is absent from the 

 pyroxene-andesites of the district, it will not serve, as has been suggested, 

 as a mineralogical distinction to separate the two natural groups, inasmuch 

 as over large basaltic areas it is wholly wanting. Moreover, within limited 

 areas, and apparently in the same flow, it may be present at one point and 

 wanting in another, occurring so irregularly disseminated through the rock 

 that any attempt to separate the basalts themselves into two divisions on a 

 basis of olivine seems futile. In an abstract of the geology of the Eureka 

 District published in 1883 this relationship between the olivine and basalt 

 was clearly pointed out. 1 Since then it has been shown that olivine is 

 absent in numerous basaltic lavas of the Great Basin. 2 Mr. George F. 

 Becker 3 has recently arrived at the conclusion that olivine can not be used 

 as a basis of division for the basalts along the sierra of California. 



characteristic Basalts. It is well to mention two other marked peculi- 

 arities of these basalts one, the very varying amount of silica which 

 they carry; the other, the very high percentage of silica contained 

 in the rock as compared with the most basaltic flows elsewhere. In 

 their chemical composition nearly all these rocks possess far more silica 



1 Third annual report of the Director of the U. S. Geological Survey, 1881-'82. 



a Arnold Hague and Jos. P. Iddiugs: Notes on the volcanic rocks of the Great Basin. Am. Jour. 

 Sci., June, 1884, p. 157. 



"Geology of the quicksilver deposits of the Pacific Slope. Monograph XIII, I'. S. Geol. Survey. 

 1888, p. 157. 



MON XX 17 



