650 ERUPTIVE BOCKS FROM MONTANA— MEREILL. voL.xvn. 



The rock (Ko. 62410, U.S.N.M.) is evidently identical with the 

 main erui^tive at Cottonwood Creek, Gallatin County, some 6 miles to 

 the southeast, and which was called an augite porphyrite in Bulletin 

 No. 110, U. S. Geological Survey. (See above.) Like that rock, it is 

 dark greenish and at times nearly black in the least decomposed 

 samj)les and thickly studded with stout idiomorphic augites of all sizes 

 up to 10 mm. in length. Near the line of contact the rock is almost 

 aphaiiitic, but shows under the microscope abundant porphyritic 

 augites and plagioclases in good idiomorphic forms in a felsitic base. 

 Eeceding from the line of contact the rock grows gradually coarser, 

 and thin sections show the rate of cooling to have been sufficiently 

 slow for an abundant development of a second generation of plagio- 

 clases. Whether any glassy base remained can not now be determined, 

 as everything is obscured by decomposition f)roducts. As with the 

 Cottonwood Creek rock, there are abundant iron oxides in large grains, 

 numerous small scales of dirty brown mica and occasional apatites. 

 The augites occur in simple, and twinned and in clustered glomero-por- 

 pJiyritic forms. 



Quite a number of the porphyritic feldspars show beautiful zonal 

 structure and no twinning. Such are assumed to be sanidins, an 

 assumption borne out in the Cottonwood Creek rock by the high per- 

 centage of potash shown in the analysis. The microstructure varies 

 from hypocrystalline porphyritic to holocrystalline pori)hyritic with a 

 panidiomorphi c grou ii d m ass. 



The only difterence which can be considered at all essential between 

 this rock and that of Cottonwood Creek lies in the development in the 

 former of abundant olivines, which, however, are now recognizable only 

 by the outlines of the dirty yellow brown chloritic decomposition prod- 

 ucts. A few of these were present in the Cottonwood Creek samples, 

 but they were so scattering as to be deemed nonessential. 



Hypersthene andesite. — Northwest of Red Bluff. This is a very fine- 

 grained and compact nearly black rock (No. 66929 U.S.N.M.) breaking 

 with an irregular choncoidal fracture and in which none of the constit- 

 uents are developed in such size as to be determined by the unaided eye. 



In the thin section the rock shows an amorphous, glassy base so 

 charged with opacite dust as to be itself almost black and opaque, and 

 bearing very numerous irregularly lath- shaped plagioclases and abun- 

 dant crystals of a colorless pyroxene. More rarely occur olivines which 

 are in all cases altered to a greenish yellow chloritic product. 



The plagioclases are many of them imperfectly secreted from the base 

 and their borders are thickly charged with the black opacite. The 

 pyroxenic mineral is in nearly colorless, very imperfectly outlined elon- 

 gated forms, often broken transversely and rarely of such size as to 

 show in basal sections prismatic cleavage lines cutting at nearly right 

 angles. 



The dichroism is very faint and in the larger forms only could it bo 



