sits which have common characteristics, and which constitute a
metallogenic province; also to groups of the same geologic age, which
indicate a metallogenic epoch (pp. 308-309). The association with
igneous rocks in one place might be a coincidence but its frequent
repetition can hardly be so explained. A zonal arrangement of minerals
about intrusives is often noted. Geologic evidence often shows the
processes of ore deposition to have been complete before the next
succeeding geologic event,--as for instance in the Tonopah district of
Nevada (p. 236), where the ores have been formed in relation to certain
volcanic flows and have been covered by later flows not carrying ore,
without any considerable erosion interval between the two events.

(2) The general contrast in mineralogical and chemical composition,
texture, and mineral associations, between these ore minerals and the
minerals known to be formed by ordinary surficial agencies under
ordinary temperatures. The latter carry distinctive evidences of their
origin. When, therefore, a mineral group is found which shows
contrasting evidences, it is clear that some other agencies have been at
work; and the natural assumption is that the solutions were hot rather
than cold; that they came from below rather than above.

(3) The contrast between the character and composition of these ores
(and their associated gangue) and the character and composition of the
wall rocks, together with the absence of leaching of the wall rocks,
favor the conclusion that the ore minerals are foreign substances
introduced from extraneous sources. The source not being apparent above
and the processes there observed not being of a kind to produce these
results, it is concluded that the depositing solutions were hot and came
from below.

(4) The fact that many of the ore minerals are never known to develop
under ordinary temperatures at the surface. For some of them,
experimental work has also indicated high temperature as a requisite to
their formation.

Quartz, which is a common associate of the ores and often constitutes
the principal gangue, serves as a geologic thermometer in that it
possesses an inversion point or temperature above which it crystallizes
in a certain form, below which in another. In deposits of this class it
has often been found to crystallize at the higher temperatures.

The quartz sometimes shows bubbles containing liquid, gas, and small
heavy crystals, probably of ferr