662 



ERUPTIVE BOCKS FROM MONTANA— MEBBTLL. 



also exposed in the upper portion of Jackass Creek, together with dikes 

 of diabase. 



Pyroxenite ; ]febsteHte. — Well up on the hills nortli of the first basin 

 of Jackass Creek was found another iuconspicaous outcrop-of a doubt- 

 fully eruptive ro(;k somewluit similar in its mode of occurrence to the 

 pyroxeiiites already described. In the hand specimen the rock (No. 

 G2442, U.S.ISr.M.) shows to the unaided eye a granular aggregate of 

 dark bronze, gray, and green minerals suggestive at once of a pyrox- 

 enite. Thin sections under the microscope show a fine granular aggre- 

 gate of light-green dialhige and colorless enstatite with included folia 

 ol brown mica, and occasional interstitial areas of lime soda feldspars 

 and more rarely a colorless biaxial mineral, showing in i>olariz:ed light the 

 wavy banding characteristic of intergrowths of orthoclase and albite. 

 Ad attempt at isolating this mineral for microchemical tests proved 

 unsuccessful owing to its small quantity. The colorless miiieral tluis 

 obtained gave always, when evaiiorated with hydrofluorsilicic acid, 

 abundant beantilully perfect hexagonal forms of sodium sflico-fluoride 

 and stellate groups of calcium silico-fluoride,but no potash salts so far as 

 could be observed. The percentage of potash shown in the bulk analy- 

 sis is, however, suggestive of some other potash-bearing mineral than 

 the mica, though it is of course possible it may come in part from the 

 pyroxenes. The diallage is but faintly green and at times almost col- 

 orless in the section, and with ditficnlty distinguished at all times from 

 the enstatite. IJesides the usual i)rismatic cleavage it shows distiuct 

 partings parallel witli both cc P ^j and ccP cc. In longitudinal sections 

 it is at times (juite fibrous, but carries no inclusions of note except the 

 brown mica. The enstatite is almost perfectly colorless and never notice- 

 ably dichroic. Enstatite and diallage make up the main mass of the 

 rock, the former being more conspicuous in the hand specimen. 



The rock nuist evidently be classed with the pyroxenites, though 

 showing transitional tendencies toward gabbro. 



Bulk analysis yielded me results as below. The ignition with sodiuku 

 carbonate indicated the presence of manganese, but which was not 

 determined quantitatively. Chromium and other rarer elements not 

 looked for. All iron calculated as FeO. 



SiO, . 

 A 1,03 

 FeO . 

 CaO . 

 MgO 

 K,0 . 

 Na,0 



Specific gravity in bulk 



54.12 

 7.91 



12.87 

 6.21 



16.64 



1.19 



.44 



99.38 

 3. 30 



