196 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
time, so that fissures could have been formed in it. But we believe that the wide¬ 
spread rock alteration in the breccia is so closely similar to that observed in veins 
that it was in all probability caused by the same or very similar waters, which we 
believe did not gain general access to the upper part of the volcano until breaks 
caused by the settling volcanic mass afforded them an ascending path. 
Among the evidence offered by Penrose to substantiate the theory of an early 
period of metasomatism is the fact that the pyrite of the altered breccia rarely con¬ 
tains gold. The same thing applies, however, practically to all of the pyrite in the 
district, which, whether in veins or in rock, seems to contain an important amount 
of gold only when admixed with tellurides. The altered breccia is, moreover, quite 
commonly cut by little seams which may be rich in crustified tellurides. In the 
massive rocks, especially syenite and latite-phonolite, where the solutions circu¬ 
lated less easily, the localizing of general pyritization near the veins is much better 
marked than in the breccia. 
EVIDENCE OF AQUEOUS ACTION. 
In their general aspect the metasomatic changes in the rocks by the vein-forming 
agencies clearly indicate aqueous solutions. As long ago pointed out by Bunsen in 
his researches on the fumaroles of Iceland, and as recognized by Penrose, 0 the 
strongest evidence of this consists in the removal of certain constituents, such as 
lime and soda, which neither are volatile alone nor enter readily into combination 
with any gaseous solvents. Further evidence is afforded by the deposition in regular 
layers shown in the seams cutting the altered rock. The widespread occurrence 
of earthy carbonates as a product of alteration also points in the same direction as 
does the general mode of attack on the rock by the solvents. We believe it to be 
firmly established that the metasomatism has been effected by aqueous solutions. 
That gases, such as carbon dioxide and hydrogen sulphide, were present in abun¬ 
dance is indicated by the general distribution of calcite, dolomite, and pyrite through 
the rocks, but these gases were present dissolved in water. The H 2 S might possibly 
have been replaced or aided by Na 2 S, though the extremely wide distribution of 
pyrite rather points to an easily diffusible gas. Both Cross and Penrose 6 express 
their belief that some volatile compound of fluorine may have been present, 
especially to effect the deposition of fluorite and the formation of the peculiar 
replacements known as granite ore. We can not share this belief, holding that both 
cases can easily be accounted for by deposition from alkaline solutions. 
OXIDATION. 
With few exceptions, and these on Beacon Hill and Poverty Gulch, the telluride 
ores near the surface are always oxidized to a brownish, claj^ey material. This 
oxidation along the veins naturally extends to far greater depth than the general 
oxidation of the country rock, and a partial oxidation may be found even at depths 
of 1,200 feet: It is very common to find brown streaks, due to limonite, following 
the narrow seams of the lode, while the surrounding breccia or massive rock 
appears entirely free from oxidation. 
" Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, p. 100. 
<> Op cit., pp. 69,161. 
