6 
ECONOMIC GEOLOGY 
liquid through the action of a pre-existing solvent: a molten 
material is liquid owing to high temperature. The problem 
is therefore whether the ore-forming solutions are given off 
from some deep layer within the earth as water vapour, 
which, after its condensation, dissolves metallic constituents 
from the ore-zone; or whether ore solutions are given off 
Sy the molten rock as a dense fluid composed of silicates 
and silica, and containing water and volatile constituents, 
Whether the ore-forming solution began as water or as 
residual molten matter squeezed out of the consolidating 
igneous rock is necessarily uncertain. The decision depends 
on the general balance of evidence given by the primary 
deep-seated ores; and the author feels that the ores due to 
hydrothermal action are more abundant and important than 
those formed as intrusive dykes of molten magma. 
Igneous ores were defined by Kemp as * excessively basic 
development of fused and cooling magmas” (Ore Deposits 
U.S., 1900, p. 59), but such ores are few and, except for 
chromite, are commercially unimportant. The aqueous 
ores include all deposits from ordinary solutions (exclusive, 
i.e. of solid solutions and molten magmas), and as most of 
these ores are due to the cooling of hot solutions, they are 
mainly hydrothermal. 
Lope Deposirion—HyparogeNEsIs, PropyrITIzATION, 
AND PNEUMATOLYSIS—The chief metallic lodes are deposited 
along great fault fissures ; and the materials depend primarily 
on the solutions that flow through the fissures.” The simplest 
process (preumato-hydrolysis or hydatogenesis) is the action 
of superheated water, which attacks the felspars, removes 
alkalis and lime, and re-deposits the other constituents as 
grains of quartz, with often some secondary felspar and such 
minerals as zoisite. The pyroxenes and amphiboles are 
broken up into granular mixtures of epidote, zoisite, and 
chlorite. The resultant rock under microscopic examination 
is a fine-grained granular mosaic in which no trace of the 
original structure is left; but the outlines of the original 
crystals and the structure of the rock may often be recognized 
in hand specimens and in examination under the micro- 
scope in ordinary light. Igneous rocks thus changed have an 
unusual lustre and were described by von Richthofen as 
bropylite ; the change was explained by Judd (0.5.6.5.