tile, and various precious stones, are
characteristically developed in pegmatites, which are known to be
igneous rocks, crystallized in the later stages of igneous intrusion.
When, therefore, such minerals are found in other ore deposits an
igneous source is a plausible inference. For instance, in the copper
veins of Butte, Montana (p. 201), are found cassiterite (tin oxide) and
tungsten minerals. Their presence, therefore, adds another item to the
evidence of a hot-water source from below.

(6) The occasional existence of hot springs in the vicinity of these ore
deposits. Where hot springs are of recent age they may suggest by their
heat, steady flow, and mineral content, that they are originating from
emanations from the still cooling magmas. In the Tonopah camp (p. 236),
cold and hot springs exist side by side, exhibiting such contrasts as to
suggest that some are due to ordinary circulation from the surface and
that others may have a deep source below in the cooling igneous rocks.
This evidence is not conclusive. Hot springs in general fail to show
evidence of ore deposition on any scale approximating that which must
have been involved in the formation of this class of ore bodies. Much
has been made of the slight amounts of metallic minerals found in a few
hot springs, but the mineral content is small and the conclusion by no
means certain that the waters are primary waters from the cooling of
igneous rocks below.

In this connection the mercury deposits of California (p. 259),
contribute a unique line of evidence. In areas of recent lavas, mercuric
sulphide (cinnabar) is actually being deposited from hot springs of
supposed magmatic origin, the waters of which carry sodium carbonate,
sodium sulphide, and hydrogen sulphide,--a chemical combination known
experimentally to dissolve mercury sulphide. The oxidation and
neutralization of these hot-spring solutions near the surface throws out
the mercury sulphide. At the same time the sulphuric acid thus formed
extensively leaches and bleaches the surrounding rocks. Such bleaching
is common about mercury deposits. When it is remembered that the mercury
deposits contain minor amounts of gold and silver and sulphides of other
metals; that they are closely associated with gold and silver deposits;
and further that such gold, silver, and other sulphide deposits often
contain minor amounts of mercury,--it is easy to assume the possibility
that these minerals may likewise have