672 



ERUPTIVE BOCKS FROM MONTANA— MERRILL. 



separated from one another by narrow intervals of feldspathic ground- 

 mass. The usual prismatic cleavage is Avell developed. 



Abundant long, needle-like light smoky crystals of apatite and the 

 usual scatteringof iron ores are also present. Scattered throughout the 

 slide are numerous irregular, triangular, or occasionally, nearly rectan- 

 gular areas of a colorless, isotropic mineral, without cleavage and 

 traversed only by an irregular network of fracture lines along which a 

 faintly greenish chloritic alteration had set in. The microscope alone 

 proving insufficient for its exact determination, chemical means were 

 resorted to. The powdered rock, treated on a slide with concentrated 

 hydrochloric acid, shortly yielded abundant cubes of sodium chloride; 

 when boiled with the acid it also yielded a jelly. That the mineral 

 was not nepheline was indicated by its optical properties. A test was 

 therefore made for chlorine by warming the powder in a platinum 

 crucible with sulphuric acid and catching the fumes arising in a drop 

 of water suspended on the underside of a covered glass. Tested with 

 nitrate of silver this drop showed an unmistakably white cloud, prov- 

 ing beyond all doubt the presence of chlorine. The isotropic character 

 of the mineral, together with its gelatinization and property of yielding 

 sodium-chloride cubes with hydrochloric acid, and chlorine by the last 

 test, all seem to point conclusively to a mineral of the sodalite group. 



Although I have spoken of the rock as granitic this structure does 

 not hold through all parts of th6 mass. Certain specimens (Ko. 38600, 

 U.S.N.M.) from near the upper contact, and from distant outcrops 

 {see p. 045), are line grained and show in the thin section a groundmass 

 with a pronounced plumose or dendritic structure more nearly like 

 that of the trachytes. 



Bulk analysis of this rock (No. 73169, U.S.N.M.) yielded me the 

 results given in column i, below. In column ii is shown the composi- 

 tion of a sodalite syenite from Square Butte, Montana, as given by 

 Liudgren.* 



* Eruptive rocks from Montana, 

 pp. 45-47. 



Proc. California Academy of Science, Vol. iii, 



