No. 374] REVIEWS OF RECENT LITERATURE. 137 
The amphibolites are “massive or schistose rocks composed 
chiefly of hornblende, usually with smaller quantities of quartz, 
feldspar, epidote, and chlorite.” They are in most cases dynami- 
cally metamorphosed diabases or porphyrites. 
The sedimentary rocks of the district are siliceous argillites, clay- 
slates, quartzites, and micaceous schists. These are altered by both 
dynamic and contact metamorphism. 
The metamorphic processes, excluding weathering, are divided 
into: (1) dynamic metamorphism, including dynamo-chemical meta- 
morphism, as in the case of the formation of amphibolites from 
diabases ; (2) common hydro-metamorphism produced at low tem- 
peratures; (3) hydro-thermal metamorphism, including solfataric 
metamorphism, and (4) contact metamorphism. The most important 
characteristic of dynamo-chemically metamorphosed rocks is the 
production of mosaics. Feldspars are among the most important of 
the new minerals formed by this process. In hydro-metamorphism 
the original constituents of rocks are broken down into aggregates 
with the production mainly of hydrated minerals. 
In his discussion of the gold-quartz veins the author calls atten- 
tion to the fact that the wall rocks of the veins have been much 
altered by metasomatic processes. The changes effected in them 
consist mainly in the introduction of carbon dioxide, sulphur, and 
potassium and the abstraction of silica and sodium. The changes 
produced in a granodiorite by these processes have resulted in a new 
rock composed of: sericite = 61.11%, quartz = 25.00%, sphene 
= .60%, apatite = .46%, pyrite=2.87%, FeCO;=.58%, MgCO, 
= 2.70%, and CaCO; = 7.23%. A siliceous argillite, originally con- 
sisting of a fine-grained aggregate of quartz, feldspar, brown mica, 
pyrrhotite, and organic matter, has been changed to an aggregate of 
sericite, calcite, and residuary quartz. The principal results of the 
interaction of the wall rock of the veins and the liquids emanating 
from the vein fissures are thus seen to be carbonates and sericite. 
The Rocks of Castle Mountain, Mont. — The Castle Mountain 
mass in Central Montana is an eroded volcano, which presents “ all 
the different types of crystallization and structure possible for an 
igneous magma to assume under the most varied conditions of cool- 
ing and pressure.” In general, the rocks have been derived from a 
siliceous magma rich in alumina and the alkalies. This has given 
rise to the various members of the granite-rhyolite family in the 
district. Associated with these, but in much smaller amounts, are 
