638 EEUPTIVE ROCKS FROM MONTANA— MERRILL. vol. xvii. 



Enstaiite andesite. — Head of small creek west of Salesville, west 

 side of Gallatin Eiver. A dense, dark-brownisli, nearly black rock, 

 without macroscopic constituents of such dimensions as to be evident 

 to the unaided eye. In thin sections, a dense, partially devitrified base 

 filled with opaque granules of iron ore, pyroxene, and feldspar micro- 

 lites, and carrying abundant small elongated phenocrysts of nearly 

 colorless pyroxenes. These without evident pleochroism, and ortho- 

 rhombic in crystallization. Hence, doubtless, enstatite. Earely small 

 augites occur. In a few instances the latter mineral occurs in the form 

 of a narrow zone about the enstatites, as described by Iddings.* The 

 feldspathic constituents are confined wholly to microscopic forms in 

 the ground mass. 



Basalt (f). — Snmll outcrop in Cretaceous, some 2i miles southeast from 

 Bozeman, east side of Bozeman Creek. 



Macroscopically the rock (Xo. 38598, U.S.N.M.) is compact, dull, dark- 

 green, almost black, thickly studded with rounded olivine in sizes up to 

 5mm. in greatest diameter, and numerous smaller green augites; none of 

 these poriDhyritic constituents are prominently noticeable, owing to the 

 similarity of their colors to that of the rock containing thera. A chlo- 

 ritic alteration has set in, attacking both the minerals mentioned as well 

 as the groundmass, and this, together with the other features men- 

 tioned, imparts to the stone the appearance of an olivine rich perido- 

 tite in which the process of serpentiiiization has far advanced. The 

 olivines in alteration have sometimes given rise to deep red ferruginous 

 products which are visible to the uiuiided eye. 



As viewed in the thin section and by ordinary light the rock consists 

 of a clear, colorless, groundmass with an illy defined radiate structure, 

 often pierced in every direction by innumerable minute needle like col- 

 orless forms, and bearing abundant black granules of iron oxides, 

 through which are interspersed countless small, idiomorphic, very light- 

 greenish pyroxenes. Abundantly distributed throughout this ground- 

 mass are the larger olivines and less abundant augites already noted. 

 A chloritic alteration has set in, attacking the augites, olivines, and 

 colorless groundmass alike, though the augites are the least attacked. 

 The most striking feature of the rock is this colorless groundmass, 

 which appears under a low power (80 diameters) and between crossed 

 nicols, as illy defined fan-shaped aggregates of elongated crystals, over 

 which the dark wave sweeps gradually as the stage is revolved. There 

 is apparently little, if any, true amorphous, glassy, or felsitic base, or 

 microlitic matter. The field, on the contrary, between crossed nicols, 

 breaks up into somewhat illy detlned polygonal areas, which become 

 light and dark as the stage is revolved, but in no case give satisfactory 

 extinction angles or interference figures. The stru-cture, in short, is 

 that of an imperfect radial spheriilitic aggregate, such as is common 



* Eruptive Eocks of Electric Peak and Sejiiilclire Mountain. Ann. Rep. U. S. 

 Geol. Survey, 1890-'91. 



