176 The American Geologist. September, 1902 
That thcv are the result of the foHation of the schists is g-ener- 
ally accepted. Where the lenses are parallel to the foliation 
there is reason to believe that they are a result of the spreading 
apart of the folia by movement. When the quartz lenticules 
lie across the vein, as they often do in the Carolinian veins, 
presenting the structure figured by Rickard* for tt;e Victorian 
veins, it seems that the quartz-filled spaces result from a rend- 
ing of the rock between two parallel fissures. Many of the 
minor peculiarities of vein-walls are due to the character of 
the rocks, but these are not intended to be treated here. Rick- 
ard has already descril^ed many features of vein-walls due to 
var}'ing rocks. 
The study of a large number of mines all over the country 
shows that, although no rule can be laid down for all localities, 
each .district will present certain peculiarities. In Montana, 
ihe rhvolytes are not favorable for well-defined constant veins, 
as the rock is too easily shattered. The granular rocks vary 
in efifect ; and the more basic forms, carrying augite or horn- 
blende, are favorable for well-defined fissures. 
As it is not my intention to do more than call attention to 
the diflferences in fissuring in varying rocks, no further men- 
tion of such peculiarities will be presented here. It is evident, 
however, that where a single mass of rock varies in texture, 
as, for example, the granite core of Castle mountain,! Mon- 
tana, in which continuous exposures show the rock passing 
from granite througli intermediate gradations into rhyolyte- 
prophyry, the vein-fissures will A'arv with the ])hysical charac- 
ters of the rock. 
II. IXFLUEXCE 0\- COUXTRV-ROCK OX \"eIX-FiLLIXG. 
Where a vein, passing through two difiterent rocks, carries 
one set of ore- and gangue-minerals in one rock, and another 
set in the other, there is a strong presumption that the varia- 
tion in the enclosing rock has caused the variation in the vein- 
materials. Similarh', if in given districts r-ue Sv^t of vein- 
minerals always occurs with a certain kind of country-rock, 
and another set with another kind, it is probable that such 
association i& genetic, and not accidental. 
• Trans., xxi., 686-713. 
+ Wred and Pirsso.v, "Geologv of the Castle Mruntain Mininp; District," 
Bull. 159, U S. G. &'. 
