646 ERUPTIVE ROCKS FROM MONTANA— MERRILL. vol.xvii. 



Augite porjyhyrite. — This rock (I^To. 38596, U.S.N.M.) as displayed in 

 the deep ravine of Cottonwood Creek is dark-gray and coarsely por- 

 pbyritic, consisting of large and very perfect coal-black augites embed- 

 ded in a dark-gray, almost holocrystalliue feldspathic groundmass. 

 Toward tlie central portions of the sheet the mass is mnch the more 

 coarsely crystalline, and through a kaolinizing of the feldspars falls 

 away to a coarse sand. From this sand were picked out in considera- 

 ble number, still fresh augites in sizes up to eight mm. in length. These 

 are usually elongated in the direction of the vertical axis, though some- 

 times in short and stout forms of a diameter fully equal to their length, 

 the crystals having the common form ooP;ccP(x;oDPa), and P. 

 Twin forms are also common, the more abundant form being that in 

 which ooPq^ is the twinning plane; more rarely occur knee-shaped and 

 clustered forms, evidently twinned after — P oc and P^. 



Ax)proaching both upper and lower contact, the rock gradually 

 becomes firmer and more compact until at last the groundmass is quite 

 aphanitic, though the porphyritic augites still retain their relative size 

 "and abundance, appearing on a treshly broken surface of a light, sage- 

 green color. At the line of contact with the shale the rock has the 

 appearance of a brownish, amorphous base, thickly sprinkled with 

 porphyritic augites and feldspars closely cemented to the shale. 



Under the microscope the coarser portion of the rock shows an 

 almost holo-crystalline groundmass of lath-shaped feldspars, small 

 augites, scales of brown mica, iron ores, and a large amount of second- 

 ary chloritic matter and calcite in which are embedded porphyritic 

 plagioclase feldspars and the large idioraorphic augites already noted. 

 The amount of uuiudividualized base is very small, and is represented 

 only here and there by small, wedge-shaped areas of greenish, chloritic, 

 decomposition products. In places these are wholly lacking, and the 

 rock assumes the panidiomorphic structure of a diabase. 



The porphyritic augites, as seen in the section, are of a very light- 

 green color, not perceptibly pleochroic, and give extinction angles on 

 clinapinacoidal sections as high as 43°. They carry inclosures of iron 

 ore, brown mica, apatite, and glass. The feldspars belonging to the 

 first generation, that is, the porphyritic forms, are all triclinic, with 

 step-like ends and abundant twin striae. They are somewhat decom- 

 posed, giving rise to chloritic and other secondary products. 



The only other porphyritic constituent ' is a greatly decomposed 

 olivine occurring in widely scattered forms, and evidently a nonessential 

 constituent. The decomposition has gone so far that in the majority of 

 cases the resultant forms are no longer recognizable. In a few 

 instances the crystal outlines are still preserved and show steep domal 

 faces and irregular fracture lines, unmistakably those of olivine. The 

 product of the decomposition is in some cases a dull green, chloritic 

 mineral; in others, a dull red, ferruginous amorphous product, accom- 

 panied in both cases by abundant calcite. The latter forms are 



