192 REVIEWS 
boron, iron, soda, and silica. Recrystallization cannot be relied upon 
as the sole explanation of the observed development of silicates and 
other contact minerals; they are due in part to a transfer from mag- 
matic sources. The ores of the district are genetically related to the 
intrusives, their deposition representing the closing stage of igneous 
activity. 
The first discovery of placer gold in Montana was made within the 
Philipsburg quadrangle in 1852. The gravels have yielded something 
less than $2,000,000. The total production of the underground mines, 
developed later, is about $50,000,000. Of this amount one-fifth is gold, 
the remainder silver. The deposits are of three types: fissure veins 
cutting both igneous and sedimentary rocks, contact metamorphic 
replacement deposits in limestone near the granite intrusives, and 
replacement deposits in sedimentary rocks. 
Silver-bearing veins in granite are of principal importance, the 
Granite-Bimetallic mines having yielded $32,000,000. The veins 
follow strong, sharply defined fissures. The wall rock shows strong 
hydrothermal alteration of the sericite-calcite type. The primary ore 
has a gangue of quartz, calcite, and rhodochrosite inclosing sulphides— 
pyrite, stibnite, tetrahedrite, tennantite, galena, arsenopyrite, and 
sphalerite. Later the veins were fractured and refilled with calcite 
and rhodochrosite. Secondary sulphides, chiefly pyrargyrite, are con- 
spicuously developed between the 300- and 800-foot levels. Oxidized 
ores containing cerargyrite, pyromorphite, and native silver occur above 
this zone. 
At the Cable mine a contact metamorphic gold copper ore occurs 
in a tabular mass of limestone surrounded by granodiorite. The ore 
replaces limestone and consists of coarse calcite and quartz with pyrite, 
pyrrhotite, arsenopyrite, magnetite, and gold. The typical ore is not 
an intergrowth with heavy contact silicates. Those minerals formed 
before the principal ore deposition began. 
Gold-bearing replacement veins in limestones near intrusive con- 
tacts form a transitional type. The minerals of the ore are quartz, 
calcite, siderite, and some pyrrhotite, magnetite, and specularite. 
Pyrite is the principal auriferous mineral. 
This report deserves a high place among recent economic papers. 
ERs: 
