I 
CHAPTER XI—GENESIS OF THE DEPOSITS AND PRACTICAL 
CONCLUSIONS. 
ORIGIN OF THE ORES. 
COMPOSITION OF THE VEIN-FORMING WATERS. 
GENERAL STATEMENT. 
From the metasomatic action, from the fissure fillings, and from other data 
certain conclusions may be drawn, regarding the characters of the vein-forming 
solutions, although our imperfect knowledge of the chemical activity of thermal 
waters under the influence of high temperature and pressure is as yet an obstacle 
to the clear understanding of their work in depth. This is a field in which intelligent 
experimentation will yet accomplish substantial progress. 
The general alkaline character of the waters with strong percentages of hydro¬ 
gen sulphide, silica, alkaline carbonates, and sulphates may be considered as firmly 
established by the following considerations: 
SILICA. 
From the presence of quartz as the most abundant gangue mineral, we may 
safely infer that silica was present in the vein-forming solutions. Chemists gen¬ 
erally agree that silica exists in hot mineral waters as Si0 2 rather than as an acid or 
as an alkaline silicate. Moreover, the substance is believed to be present in a colloid 
solution. Colloids are supposed to exist in solutions as suspended particles and 
they exhibit certain striking differences from electrolytes in solution, especially in 
their mode of diffusion. They do not diffuse through semipermeable membranes 
or walls through which electrolytes, such as ordinary salts, may freely pass; on 
the other hand, electrolytes freely diffuse through colloid solutions and jellies, 
which may be considered as concentrated solutions of colloids. These facts help 
to explain the rare occurrence of silicification in the wall rocks of quartz veins, 
the colloid solutions of silica being held by the semipermeable rock walls, while 
other constituents and gases in solution may more or less freely permeate them. 
The derivation of the silica is a difficult matter to ascertain. One may conceive 
of its being dissolved by atmospheric waters from the granite, from the breccia, or 
from the various phonolitic rocks. The latter contain, however, extremely little 
free quartz, and almost the only way in which ascending waters could abstract 
silica from it would be by decomposition of the various silicates. Some of the 
silica in the waters has certainly this origin, but it by no means follows that this is 
the only source. It is quite possible that the phonolitic magma cooling at great 
depths, gave off some* silica, together with some of the magmatic water, which it 
may have contained. 
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