Influence of Country Rock. — Weed. 171 
and chemistry, rock-determinations are now scientifically made, 
while the examination of thin sections of ores supplements 
field and laboratorv study, and affords conclusive evidence of 
the paragenesis of the ore-minerals. 
In the great m.ass of data, regarding mineral deposits, accu- 
mulated during the past century, there are some facts which 
stand the test as to reliability; but the greater part are not 
available for use in this discussion. Within the last twenty 
years, however, many able workers have contributed careful 
and accurate accounts of various ore-deposits ; and it is bv the 
facts given in such papers, and by personal experience, that I 
have become convinced that there is in some districts a true 
relation between the country-rock and the mineral contents of 
the veins. 
Aly own interest in the subject arose from the observation 
of a number of striking instances of the variation of vein-con- 
tents with the nature of the enclosing rock. Having already 
recorded certain observations of this kind, I venture to present 
in this paper further notes, together with a few facts from the 
literature of the subject, which seem to prove the correctness 
of my view. It is apparent that if the relation holds true, 
though in limited districts only, it will be of great practical in- 
terest to the miner, and will confer upon the geological survey 
of a mining district a new value, rendering it, in fact, an almost 
indisoensable preliminarv to the extensive working of large 
properties. 
The intiuence of the enclosing rocks upon the mineral-veins 
evidentl}' affects both the vein-structure and the vein-filling. 
The first effect is physical ; the second is mineralogical or, pri- 
marily, chemical. The first has been but lightly tC'Uched vtpon, 
if -noticed at all, by writers on mining geology. The second has 
long been recognized as a fact in a few well-known examples ; 
but, although many attempts have been made to show a rela- 
tionship between certain ores and definite rock-types, the corre- 
lations have been of local value only. 
I. Influence of Rocks on A^ein-Structure. 
It is self-evident that in most mineral deposits the rocks must 
have been either porous or so fractured as to permit the circula- 
tion of underground waters as a preliminary to vein-formation. 
It is also evident that accordino- to the varving hardness, tough- 
