1894. 



PEOCEEDIXGS OF THE XATIOXAL MUSEUM. 



659 



with the oliviues. The spinels are so-abiiudaiit as to be the most.striking 

 feature of the slide and are readily recognizable by the unaided eye, in 

 the form of irregular oi)aque granules a luillinieter or so iii greatest 

 diameter. 



From the powdered rock the spinel was sejiarated out by digestion 

 with hydroHuoric acid. The material thus obtained yielded Mr. Eakius 

 results as below : 



This reduces readily to the spinel formula (MgjFejO.AlaOj, which is 

 that of the variety pleonast, to which tiie mineral has already been 

 referred. 



A bulk analysis of the rock yielded me results as below, no attempts 

 being made at determining the rarer constituents: 



SiO, . 

 Al.Oj 

 Fe.,0., 

 MgO 

 CaO . 



Diabase. — Granite Creek, Madison County. These are coarsely crys- 

 talline rocks (No. (32403, U.S.N.M.), in most cases readily recogniza- 

 ble in the field as diabases, though in some instances the uralitization of 

 the augitic constituent had gone so far that in the hand specimen the 

 rock might easily be mistaken for a diorite. In the thin section they 

 present nothing of special interest. The ophitic structure characteris- 

 tic of diabases is not iirominently developed. There are broad areas 

 of badly kaolinized ])lagioclases interspersed with augites, iron ore, and 

 occasional quartz granules and shreds of brown mica, together with 

 more or less uralitic hornblende and chlorite. 



Basalt. — The high iiat-topped plateau northeast and east of Virginia 

 City is composed exclusively of basalt with interbedded tufts, the 

 whole being underlaid by andesites, which are exposed only in the dry 

 gulches down well toward the level of the town (Nos. 62405 and 

 6240G, U.S.N.M.). The basalts vary m color from dull reddish to 



* All iron determiued at Fe.Oa. 



