CHANGES IN ORE SINCE DEPOSITION. 241 
tion shows that the change has been brought about by waters which ci: cu- 
lated along the Castle Creek fault. It is likely that soluble sulphates, 
brought up by hot-spring or other waters, were precipitated when coming 
in contact with carbonate of lime, as gypsum, and thus a true replacement 
was effected. 
Leaching of rocks—Along the underground channels the effects of circulat- 
ing waters are seen in the softening of the rocks through which they pass. 
The process consists in the leaching out of the more soluble constituents 
and in the reduction of the comparatively insoluble portions, by removal of 
the cement, to a clayey form. Thus in the vicinity of the Castle Creek 
fault in Keno Gulch granite is altered to a kaolin which contains grains of 
residual quartz. Along the Lenado fault in the Aspen Contact and Lead- 
ville mines the Cambrian quartzite is softened for some distance so as to 
form a white clay, which grows coarser and more solid as the distance from 
the fault increases. This clay is often called by the miners “ porphyry,” 
- but the process of its formation seems to be the removal of some of the 
silica, especially the secondary cement between the quartz grains, by circu- 
lating waters. When the cement is removed the rock disintegrates and 
forms a clay. 
The alteration of limestone and dolomite by surface waters along 
faults and fracture zones is seen throughout the mineral-bearing district. 
Thus along the Silver fault in certain mines the limestone becomes altered, 
so as to be mistaken for Weber shales, although the Weber rocks them- 
selves are not soft and claylike except where they have been acted upon 
by these surface waters. At one locality in the Argentum-Juniata mine 
the dolomite along watercourses was observed to be altered completely for 
some distance to a soft clay, which is sometimes bleached to a yellow color, 
and goes among the miners by the name of “tale” or “shale.” The stages 
of transition between the solid dolomite and the clay show sufficiently 
the origin of the latter. 
In the hard Weber limestones in several places the same phenomena 
were observed. In the Clark tunnel at Lenado two samples were taken for 
analysis from the Weber rocks, one being of limestone which was partially 
softened and the other of limestone which was completely softened and 
also bleached. These analyses are given as Nos. 2 and 3 in the following 
MON XXxI——16 
