72 GOLD AND TIN DEPOSITS OF SOUTHERN APPALACHIANS. 
richest. The fact that in some cases the ore bodies pitch away from and leave the dikes 
ought to be good proof that the ore deposition was independent of the dikes and dike 
fissures. 
DEPOSITION OF THE ORES. 
In the consideration of this subject also the lack of analyses prevents the drawing of 
accurate and valuable conclusions. It has been shown that the ore-bearing solutions were 
probably under greater pressure and at higher temperature near their source than at the 
place of deposition. A ready explanation of the cause of deposition would be that the 
decrease in pressure and temperature lowered the solubility of the materials carried in solu- 
tion and thus resulted in their precipitation. It has been pointed out by Mr. Lindgren,^ 
however, that in general increasing temperature and pressure do not increase the solubility 
of ore minerals beyond certain maxima, and that consequently the effect of decreasing 
pressure and temperature on the deposition of ores has probably been overestimated. It 
is certain, on the other hand, that these changing conditions must have had some effect on 
the deposition of the ores, but there is no evidence by which to measure it. 
It is believed by the writer that the deposition of the ores was brought about mainly by 
a change in composition of the relatively concentrated solutions. This change in compo- 
sition would disturb the nice equilibrium which had been attained, and cause a decrease 
in solubility and consequent precipitation of some or all of the constituents. Such a 
change in composition of the solutions might be brought about in any or all of three ways: 
First, change in conditions of temperature and pressure. This would have some effect, 
probably not very great. Second, mingling with solutions from another source, such as 
atmospheric waters along cross fissures or channels. The intersection of such channels 
with the ore-bearing fissures may have fixed the location of extraordinary deposition, the pay 
shoots. While the vein solutions were doubtless under greater pressure than that which 
would be due to hydrostatic head, it is conceivable that solutions from the surface, which 
would be under hydrostatic head only, might mingle with them in spite of this difference 
of pressure, through the agencies of friction, capillarity, and diffusion. Third, the mutual 
chemical reaction of the solutions and the surrounding rocks. The subtraction of silica, 
potash, and possibly alumina from the solutions and the addition to them of soda and lime, 
which have taken place in nearly or quite all cases, would exert a very pronounced influ- 
ence on the solubility of the remaining substances and cause their precipitation. It is 
believed that this third factor was the most important in the deposition of the ores. 
The gold and sulphides in the veins have probably been precipitated in part mechanic- 
ally with the quartz and in part chemically. These heavy metals have also been carried 
with the silica and lighter metallic elements into the wall rocks and there deposited, though 
less abundantly than in the fissures. 
It has been stated (p. 61) that the fissure veins and the replacement deposits probably 
had the same origin. The occurrence of similar minerals in the two types of deposits 
and the presence of deposits of intermediate character^sustain the conclusion that the 
nature of the solutions and the source of the materials were the same in each case. 
In the mode of deposition of these materials from the solutions must lie an explanation 
of the differences which exist between the two classes. Briefly stated, these differences are 
as follows: The fissure veins are narrow and relatively rich; they effect a very marked 
change in their wall rocks for a short distance away from them; they occur in dense, usually 
basic rocks, already rendered resistant through profound regional metamorphism. The 
replacement deposits are large and irregular; they consist of altered country rock, and they 
occur in porous volcanic rocks of acidic character, easily susceptible to alteration. 
The ore-bearing solutions, ascending at a high degree of concentration and temperature, 
would, on confinement to a narrow fissure, be able to exert intense action on the immedi- 
ately adjoining, not easily pervious wall rock. The same solutions, reaching a body of 
porous, easily alterable material, would immediately disseminate through it, distributing 
o Gold-quartz veins of Nevada City and Grass Valley districts, California: Seventeenth Ann. Kept. 
U. S. Geol. Survey, pt. 2, 1896, pp. 177-178, 183. 
