ORIGIN OF THE ORES. 
229 
some constituents and gain of new ones, and finally the mingling of the ascending 
solutions with the moisture already permeating the rock. The most satisfactory" 
way of attacking such a problem is to begin by experiments in which these confusing 
complications are to some extent eliminated. 
Simply from the results as we see them, without positive knowledge as to the 
composition and physical conditions of the solutions, a few conclusions may be 
reached. One of the most striking facts is the difference in composition between 
the filled veinlets and the altered country rock. The silica, the gold tellurides, 
and the sulphides (except pyrite) are retained in these fissures and precipitated in 
them. The walls are evidently not as a rule permeable for these substances. It 
seems to be generally true that in gold-quartz veins the silica, the gold, and some 
sulphides, such as galena, sphalerite, and chalcopyrite, are always deposited in open 
spaces, if such spaces are available. This rule, which may not be without exceptions, 
indicates that the wall rocks effect a separation in the solutions and that the sub¬ 
stances mentioned penetrate them less easily than the other constituents of the solu¬ 
tions. For tellurides the rule is not inflexible, for it has been shown that they also 
occur in metasomatic form. In the “granite ore/’ however, which is considered 
one of the best examples of metasomatic deposition of tellurides, most of the latter 
are probably deposited in cavities created by the solutions and consequently not, 
strictly speaking, metasomatic. It is known that tellurides in undoubted metaso¬ 
matic development occur abundantly in Kalgoorlie, Western Australia. 
This question of diffusion through porous rocks in relation to veins was first 
suggested by a microscopic study of the quicksilver ore? of California and of the 
altered wall rocks of gold-quartz veins in the same State. 
G. F. Becker" first expressed the belief that a law existed governing this phe¬ 
nomenon and wrote as follows: 
Osmotic hypothesis .—These observations would be explained if it were true in general that liquids mil 
penetrate a dense wall or septum at a sensible rate only when there is a chemico-phy ical reaction between 
the solid and the fluid. Here the term chemico-physical reaction is intended to express any chemical union 
or physical change attended by the evolution of heat, or rather by the degradation of energy. There is reason 
to think that such a law really exists, though it can not be said that it is conclusively proved. * * * 
On this hypothesis the concentration of ores in deposits would be largely due to the fact of the lack of 
action between their solutions and the wall rocks; and the decomposition of the country rock, so often observed 
near veins, would be due to the absorption of solutions of gangue minerals by the walls. In short, there would 
be a species of concentration by dialysis. 
As expressed by Becker the hypothesis is probably not true, for it is known 
that solutes diffuse through chemically inactive membranes or porous bodies at 
varying rates; some are entirely impermeable while others are partly so, and the 
local osmotic pressure varies according to this permeability. But it is probably 
true that the rate of diffusion partly depends on chemical action between mem¬ 
brane and solute. The silica was in all probability contained in the waters in 
colloidal, easily soluble form, 6 and it is at least a legitimate subject for inquiry 
whether or not the sulphides and tellurides were dissolved in the same manner. 
a Quicksilver ore deposits: Mineral Resources IT. S. for 1892, U. S. Geol. Survey, 1893, p. 156. 
b Lindgren, W., Gold-quartz veins of Nevada City and Grass Valley, Cal.: Seventeenth Ann. Rept. U. S. Geol. Survey 
pt. 2, 1896, p. 183. 
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