Influence of Country Rock. — JVccd. lyy 
For many years there has been a widespread behef among 
mining geologists and engineers that igneous rocks are an al- 
most invariable accompaniment of productive ore-deposits of 
the precious metals. Whether the igneous rocks be regarded 
as the actual source of the metal, or as the cause of Assuring 
and ore deposition by reason of dynamic disturbance, the re- 
sult is genetically due to the volcanic forces. Prof. Vogt has 
recently given us, in his able and instructive paper, f a resume 
of his studies, and Prof. Kemp has shownt both the competency 
of the igneous rocks themselves as a source of supply and of 
the intrusives as a source of energy. It is not this phase of 
the subject that I propose to discuss, but the time-honored 
question of the influence of wall-rock upon the mineral con- 
tents of the vein. Many students of ore-deposits, familiar with 
the common occurrence of galena-ores in limestone, and the 
changes in mineral character of the Cornish veins with change 
of rock have sought to establish a relation betvveen certain 
rocks and certain ores. While such a condition s^:ems to pie- 
vail in a few districts, no general law has been established by 
these attempts. 
Conditions Governing the Relation of Country-Rock to l^ein- 
Contents. 
In considering the relations between country-rock and vein- 
contents, the following premises are assumed : 
1. Vein-filling may be tlie result of (a) the filling of open 
fissures, (7') of replacement, or {c) of both filling and replace- 
ment. 
2. The ore- and gangue-minerals of all these types vary. 
Lindgren has divided veins filled by metasomatic replacement 
into eleven classes, and shows that the chemical processes in- 
volved were very different in each case. It is made certain 
by the study of altered wall-rocks and of mine-waters that 
the mineral-forming solutions have varied greatly in character. 
Moreover some veins have been opened after formation, and 
new minerals have been introduced by later solutions. 
In veins the material of which is well crustified, or is 
known to be the result of the filling of open fissures, a marked 
influence of the wall-rock on the contents of the vein would 
+ "Problems in the Geolojiy of Ore-Ueposits," Trans., xxxi., 125. 
t "Role of the Igneous Rocks," etc., Trans., xxxi., 1<59. 
