amounts of tungsten are being obtained from the primary or
fixed deposits. These are found almost exclusively in association with
granitic rocks, and have a variety of forms. The most productive
deposits are in the form of veins, cutting the granites and the
surrounding rocks into which the granites were intruded, and containing
quartz, metallic sulphides, and in some cases minerals of tin, gold, and
silver. The deposits of the two most important districts in the United
States, in Boulder County, Colorado, and at Atolia, California, are of
this general nature. The close association of such deposits with
plutonic igneous rocks, and the characteristic mineral associations (see
pp. 37-41) suggest strongly that the deposits were formed by hot
solutions deriving their material from a magmatic source.

Other tungsten deposits, which only recently became of importance, are
of the contact-metamorphic type--in limestones which have been invaded
by hot aqueous and gaseous solutions near the borders of granitic
intrusions. In these occurrences the tungsten mineral is almost
invariably scheelite, and is associated with calcite, garnet, pyroxene,
and other silicates. A magmatic origin of the tungsten is probable. Some
of the deposits of the Great Basin area and of Japan are of this nature,
and it is believed that important deposits of this type may be
discovered in many other countries.

Tungsten is likewise found in original segregations in igneous rocks and
in pegmatite dikes, but these deposits are of comparatively small
commercial importance.

In some tungsten deposits a hydrated oxide called tungstite has been
formed as a canary-yellow coating at the surface. On the whole, however,
tungsten minerals are very resistant to weathering, and in all their
deposits secondary concentration by chemical action at the surface has
not played any appreciable part. The disappearance of tungsten minerals
from alluvial materials which are undergoing laterization, which has
been described in Burma,[32] seems to indicate that the tungsten is
dissolved in surface waters to some extent; but in the main it is
probably carried completely out of the vicinity and not reprecipitated
below.

MOLYBDENUM ORES

ECONOMIC FEATURES

The main use of molybdenum is in the manufacture of high-speed tool
steels, in which it has been used as a partial or complete substitute
for tungsten. Its steel-hardening qualities are more effective than
those of tun