oi.niNi'. IN r.ASAi/r. "i.x 



their structure a uniform <inmiuliii;iss made up of conrsi'-Lii-aiiUMl aji-<iT('<::i- 

 tions ot" feldspar and auf^iti', iml)edded in a globiditir ijlass l)ase. 



Nearly all the rocks of intermediate mineral composition ]iossess the 

 basaltic hal)it. llvpersthene is wantinjj;- in the normal l)asalts. Auyite and 

 mag-netite, although essential minerals in the composition of Ixith rocks, occur 

 much more a])undantlv in basalts. With one exception the microscope has 

 failed to detect olivine in auA' thin section of the lavas (dassed as pvroxene- 

 audesite, the excej)tion, however, furnishing quite a remarkal)le rock, and 

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

 in an ol)scure exposure or knoll in Fish ("reek \'alle\' just west of C'litl' 

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

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

 referred to the in'roxene-andesites. Although olivine is absent from tlie 

 pvroxeiu'-a'idesites of the district, it will not serve, as has been suggested, 

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

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

 areas, and apparently in the same How, it niay l)e jjresent at onc^ point and 

 wanting in another, occurring so irregulf.rlv disseminated through the rock 

 that anv attempt to se2)arate the basalts themselves into two divisions on a 

 basis of olivine seems futile. In an abstract of the geolog\' of the Eureka 

 District published in 1883 this relationship between the olivine and basalt 

 was ch'arK- pointed out.' Since then it has been shown that olivine is 

 absent in numerous basaltic lavas of the (n-eat Hasin." ]\Ir. (Jeorge F. 

 Beckei- ' lias recently arrived at the conclusion that olivine can not be used 

 as a basis of division for the basalts along the sierr;i of California. 



Characteristic Basalts.— It is Well to mentit>n two other marked peculi- 

 arities of these basalts — one, the very ^■arying amount of silica which 

 thev carr\-: the otlier, the very high percentage of silica contained 

 in the rock as compariMl with the most ])asaltic Hows elsewhere. In 

 theii" clieniical composition nearl\' all thi'se rocks possess fai" more silica 



'Third annual rejiort i>f the Director of the U. S. Geological Survey, 1881 -'82. 



•Arnold Hague and Jos. P. Iddings: Notes on the volcanic rocks of the (Jreat Hasin. Am. .lour. 

 Sci., .luue, 1884, p. 457. 



''Geology of tlii' c|uirksilviT deposits of the I'acilic Slope. Monograph xill, V. S. (Jcol. Survey, 

 1888, p. 157. 



MON XX 17 



