426 WEED AND PIRSSON 
and magnesia, as shown by the infrequency or absence of min- 
erals containing these elements and by the alkaline nature of the 
feldspars. There has been, however, a certain amount of differ- 
entiation or variation in its character and the resultant rocks 
grade from true granite-porphyries through quartz syenite-por- 
phyries into syenite-porphyry. In one case, through local 
increase in lime, they pass into a granite-diorite-porphyry. While 
the alkaline magma in general is high in silica, a local differen- 
tiation, has produced a form rich in alkalies but low in silica, as 
shown by the tinguaite. 
The results then show that petrologically the magmas of the 
Little Rockies conform to the general type of the detached 
mountain groups of central Montana in that they are of alkaline, 
highly differentiated character;* they appear to differ in one 
respect from the general characteristic of this petrographical 
province in that soda dominates the potash, though but slightly. 
The occurence of tinguaite adds another locality to the few 
already known American occurrences of phonolitic rocks. Such | 
rocks have been described from the Black Hills of South Dakota,? 
from Arkansas,3 from the Trans Pecos district, Texas,‘ from 
Cripple Creek, Colo.,5 and from the Bearpaw Mountains of Mon- 
tana,° and specimens have also been received from the Sweet 
Grass Hills of Montana. Closely related types also occur in the 
Crazy Mountains of Montana.’ 
Ore deposits—The ore deposits of the Little Rocky Moun- 
tains are of considerable scientific interest, since they represent 
a type that has thus far been noted at very few localities in this 
*Highwood Mts. Bull. Geol. Soc. America, Vol. VI, p. 389. 1895. Phonolitic 
Rocks from Montana, Am. Jour. Sci., Vol. L, 1895. Igneous Rocks of Sweet Grass 
Hills, Montana, Am. Jour. Sci., Vol. L, 1895. 
2 Pirsson, Am. Jour. Sci., Vol., XLVII, p. 341, 1894. 
3J. F. Williams, Ig. Rocks Arkansas, pp. 99, 146, 264, 277, 351, 367. 1890. 
4Osann, Geol. Surv. Texas, Ann. Rep., 1892, p. 130. 
5 Cross, Proc. Colorado Sci. Soc. 1887, p. 167. Pikes Peak Folio, Geol. Atlas, 
U. S. Geol. Surv. 1894. 
6 Am. Jour. Sci., Vol. L, p. 394. 
7 Wolff and Tarr, Bull. Mus. Comp. Zool., Cambridge, Vol, XVI, 1893, p. 230. 
