178 The American Geologist. September, 1902 
not be expected. In the majority of veins, however, there is 
evidence of more or less metasomatic replacement ; and it is 
evident that the nature of the wall-rock will be an important 
factor in the chemical reactions of the processes of replacement. 
It should be remembered, however, that the evidence of many- 
districts shows that veins of different kinds and ages may form 
in the same rock, and hence it is not to be expected that any 
general conclusions, applicable to all veins, can ever be reached. 
Examples. 
Butte, Montana. — In the typical silver-veins of this district 
the filling consists of quartz, showing well-marked "comb"- 
structure. with rhodonite, rhodochrosite, pyrite, zinc-blende 
and silver sulphides. The structure is clearly that of the 
filling of open fissures^. These veins occur in the normal Butte 
granite, a quartz-monzonyte, and in the Bluebird granite, which 
is an aplyte. There is, however, no perceptible difference in 
either the character or the tenor of the ore between the veins 
in the aplyte and those in the normal granite, or in the parts of 
the same vein where it cuts the two rocks. In the copper- veins 
on the other hand, there is a marked difference. These are of 
undoubted metasomatic origin, and were formed by the replace- 
ment of the rock along fracture-planes. In the copper-area, 
the veins cut both the two rocks previously mentioned and 
also a quartz-porphyry, which I have named the Modoc 
porphyr}-. In the Butte granite, the veins are commonly rich 
in copper ; in the Bluebird granite they are almost equally 
wide and strong, but are lean, and composed chiefly of quartz, 
with comparatively little pyrite and copper. In the porphyr}', 
the veins are narrow and lean. There are so m?.ny instances 
in which the same veins can be seen cutting all three rocks 
that there can be no doubt as to the correctness of these con- 
clusions. The vein-filling consists of quartz which shows no 
comb-structure, with pyrite and various copper-minerals, of 
which chalcocite, enargite and bornite are the most common. 
The walls on each side are much ^altered, and the less altered 
rocks at some distance from the veins show the ferromagnesian 
silicates altered to pyrite. The relative richness of the veins 
in the. Butte granite is believed to be due to the basic character 
of the rock, and its greater content of the easily replaceable 
iron silicates. The rock is a cjuartz-monzonyte, the compo- 
