1897,] Petrography. 53 
upon as a chemical one. The rock magma is supposed to have fused the 
hornblende or biotite grains, or to have partially dissolved them, and 
from the resulting mass the augite and magnetite are believed to have 
crystallized. Washington,’ in a recent article, discusses this theory. 
He shows that the alteration is confined almost exclusively to the con- 
stituents of the intermediate and basic volcanic rocks. It is not a 
phenomenon of acid rocks, nor of plutonic basic ones. The author 
believes that the two minerals named are formed under intratellurial 
conditions, and that when the conditions are changed to those prevail- 
ing at the surface the complete but homogeneous compounds break up 
into a heterogeneaus aggregate of simpler ones, i. e., becoming para- 
morphed. It is believed that many of the grains of augite and magne- 
tite scattered through certain volcanic rocks may have been components 
of these paramorphs that have been carried from their original positions 
by magma movements. Some of the augite andesites are thought to 
owe their augitic constituent to the processes above outlined. 
Petrography of the Little Rocky Mountains, Mon.—The 
Little Rocky Mountains are situated in central Montana, about 180 
miles east of the Rocky Mountains proper. They are formed by a dome- 
shaped uplift of Paleozoic and older rocks in the midst of horizontal 
cretaceous strata. The nuclear rocks are classed by Weed and Pirsson* as 
Archean-Algonkian, because consisting of various schists associated with 
a quartzite. Around these and covering them, over much of the extent 
of the mountains, are porphyries that grade in places into phenolitic 
facies. The rock was extruded asa laccolitic mass, which now partially 
covers the Archean schists. In the main it is a granite porphyry, con- 
taining orthoclase and oligoclase phenocrysts in a fine grained ground- 
mass composed almost exclusively of orthoclase, anorthoclase and 
quartz. This rock is replaced occasionally by a syenite-porphyry, or by 
a granite-diorite-porphyry, which differs from the granite-porphyry in 
the presence of chloritized augite and in the predominance of plagio- 
clase phenocrysts over orthoclastic ones. At two places tinguaite re- 
places the normal rock. This phonolitic phase is a dense, dark-green 
rock, that is apparently a contact phase of the normal porphyry. In 
their section the tinguaite shows large phenocrysts of sanidine and 
smaller ones of augite in a fine groundmass of alkali-feldspars, aegirite 
and nephelite. The syenite-porphyry from Lookout Butte is charac- 
terized by the absence of all minerals but the feldspars and a little 
3 Jour. Geol., Vol. IV, 1896, p. 257. 
‘Jour. of Geology, Vol. IV, 1896, p. 399. 
