No. 12. — Contributions from the Petrographical Laboratory of the 
Harvard University Museum. 
Mee 
Acmite Trachyte from the Crazy Mountains, Montana. By J. E. Wourr 
AND R. S. Tarr. 
In the progress 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 distinguished 
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 latter, 
and so far as known all the other eruptives,? solely as intrusive rocks. 
The field types can conveniently be classed under three heads: Ist. Dikes 
cutting the Cretacous (Laramie?) 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 the 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. 
Acmate-trachyte Type. — When occurring in the latter forms, the rock 
is of a fresh green to grayish green color, with a somewhat greasy lustre 
and a conchoidal fracture. Glassy feldspar crystals 5 or 6 mm. long 
give it a porphyritic character, and smaller augite phenocrysts are com- 
* No. IV. Metamorphism of Clastic Feldspar in Conglomerate Schist, by J. E. 
Wolff. 
1 J. E. Wolff. 
2 Ibid., “ Geology of the Crazy Mountains,” Bull. G. S. A., Vol. III. p. 445. 
VOL. XVI. —No. 12. 
