Xo. 12. — Contributions from the Pefrograpliiccd Laloratory of the 



Harvard University Musenm. 



Acmite Trachyte from the Crazy Mountains, Montana. By J. E. Wolff 



AND R. S. Tarr. 



In the prorji-ess of the field investigation of the eruptive rocks of the 

 Crazy Mountains, Montana, by one of the writers/ in 1889, the occur- 

 rence of a group of eruptive rocks was noted, which were distinguislied 

 from the theralites and associated rocks by their gray or greenish gray 

 color and somewhat greasy lustre, and of which about a dozen specimens 

 from as many localities were collected for further study. 



Field Occurrence. — They were only found in the northern half of the 

 range, in general associated with theralite, and occurring like the lattei-, 

 and so far as known all the other eruptives,^ solely as intrusive rocks. 

 The field types can convenient!}' be classed under three heads : 1st. Dikes 

 cutting the Cretacous (Laramie T) shales and sandstones ; 2d. Small in- 

 trusive sheets parallel to the bedding; 3d. Thick, bulging, laccolitic 

 sheets, which may send apophyses into the adjacent shales. In tlie last 

 form, sheets of the rock have been observed which are a hundred feet 

 thick at the bulge and a mile long, and, conforming with the tilted 

 position of the strata, produce long high ridges with sloping back and 

 steep front. As with the theralite, diorite, and other eruptives of the 

 range, the rock is coarse, almost granitic in the thick sheets, fine-grained 

 and porphyritic in the smaller sheets, dikes, and apophyses. 



Acmite-trachyte Type. — AVhen occui'ring in the latter forms, the rock 

 is of a fresh green to grayish green color, with a somewhat greasy lustre 

 and a conchoidal fractui'e. Glassy feldspar ciystals 5 or 6 mm. long- 

 give it a porphyritic character, and smaller augite phenocrysts are com- 



* No. IV. Metamorpliism of Clastic Feldspar in Conglomerate Schist, by .J. K. 

 Wolff. 



1 J. E. Wolff. 



•■2 Ibid., "Geology of tlic Crazy Mountains," Bull. G. S. A., Vol. III. p. 44-3. 



VOL X\I — NO. VI. 



