Influence of Counfry Rock. — Weed. 
179 
sition of which has been carefully calculated from chemical 
analyses of the rock, and of the biotite and hornblende isolated 
from it, and from microscopic analysis of the rock as well* 
This shows it to contain 15.26 pei" cent, of hornblende and 
4.22 of biotite. There is a little au^g^ite also. The aplyte con- 
tains over 10 per cent, more silica than the rock just noted, no 
hornblende, and very little biotite. The Modoc porphyrv also 
has, when fresh, a very little mica, and is high in silica as 
the aplyte. Mr. H. V. Winchell has called my attention to the 
condition shown in the diagram. Fig. 4. In the case here given 
as a general type, the vein is workable only in the Butte 
granite. 
A study of thin sections of the rock adjacent to the ore 
shows that the hornblende is the first mineral to be altered 
into ore, and that the bunches of this mineral form the nucleus 
Fig. 4 
_y_!xl Granite 
Aplite 
^-?^'^''^l Quartz 
Ideal Plan of Conditions in a Copper-vein at tutte, Mont., passing 
from Basic Granite into Aplyte Masses. 
for a more or less complete replacement of the entire rock. 
The general principles of this metasomatic replacement are 
those given by Lindgren.t It is probable that the alteration 
now seen in the wall-rock with its nests of pyrite replacing 
the biotite and the hornblende, may present the earlier stages 
of the metasomatic process, and that the pyrite thus formed 
was not only the nucleus for a further deposition of pyrite, 
but that the pyrite itself was the precipitating agent for the 
copper-minerals, as has been shown to be the case in the sec- 
ondary enriched ores. In the aplyte there is more qua rtz and less 
• "Granite Rocks of Butte, Mont." W. H. Whed, Jour. Geo/., April, 1900. 
t "Metasomatic Process in I'issure-Veins." Trans., xxx., 578. 
