Inftnciicc of Country Rock. — Weed. 187 
this paper. Nevertheless, some metasomatic alteration of the 
rock adjacent to the fissure is usually present; and it must 
be admitted that manv veins show evidence both of the fiUino- 
o 
of open cavities and of deposition by replacement. 
As already indicated, the coincident variation of mineral 
contents and wall-rock is due to metasomatic action, whereby 
there is a chemical interchange between the rock and the 
vein-producing solutions. The metasomatic process varies 
greatly because the solutions vary. Lindgren''^ has treated the 
stibject fully, and has indicated the chemistry of the process, 
so that a detailed account of the reactions involved is unnec- 
essary here. According to his clear demonstration, the altera- 
tion of the same kind of wall-rock shows that the vein-forming 
solutions have varied greatly in their chemical effect upon vein- 
contents, even where the veins are of metasomatic origin. 
The most frequent process seems to be, in granitic rocks, 
a reaction between the ferromagnesian minerals, such as augite, 
hornblende, biotite, etc., and the vein-forming solutions, with 
the formation of pyrites and other sulphides. If later recon- 
centration occurs, bonanzas are formed by reaction of the py- 
rites on later solutions. This process I have treated quite 
fully in a monograph now in preparation on the Butte ore- 
deposits. 
In other cases the feldspars are attacked, and the iron 
minerals of the rock are not replaced. Such a case is described 
by R. C. Hills ;t and Richard ascribes the silver-lead ores of 
Pontgibaud, France, to the replacement of the feldspars of 
granulyte, as already quoted. 
The attempt to tabulate the relation between vein-contents 
and country-rock has been made by various writers, and lately 
by Stretch. The latter author, in his very interesting essay, $ 
has given 139 occurrences, and attempted to eliminate the per- 
sonal equation of the observer. It is evident, however, that 
existing literature is not adequate for such work. Too often 
the rocks are wrongly named, or receive generic terms only. 
The words "granite" and "porphyry" have long been used 
in a textual sense by mining engineers, and by many geologists 
* "Metasomatic Processes in Fissure Veins," Trans., xxx., 578. 
t Proc. Colo. Sci. Soc, vol. i., p. 20. 
t Poeketbook for Prospecting, Locating, and Valuing Mines, Sci. Pub. Co., 
N. Y., 1900. 
