Influence of Country Rock. — JVeed. 185 
volatile compound of various metals, as supposed by Lind- 
gren, passed into solution. Concerning the occurrence of this 
class of deposits in limestone, ]\Ir. Emmons's description of 
the Greenwood ore is quite significant. He says : "The ore- 
bodies are cut by eruptive dikes that do not apparently disturb 
or exert any incfamorpliic influence on the ore, and yet are 
not at all mineralized themselves." At a number of localities 
seen by the writer, the veins, if traced into the granite or dioryte, 
would be found to be barren of galena, and generally without 
value. On the other hand, at Alarvsville, Montana, the veins 
show no appreciable change in character in passing through 
the altered shales or hornstones into the granite. It should be 
noted, however, that the ores at this place are not lerd-bearing. 
Aloreover, I am told by ^ir. G. H. Robinson, the former man- 
ager of the mine, that in the earlier workings the veins showed 
, a marked tendency to be richer in approaching the granite ( or 
rather dioryte) contact, and were poor when they passed into 
the granite. 
Thunder Bay Sihcr-P^cins.^i-Mr. H. V. Winchell tells me 
that the Thunder Rav district of Canada afifords excellent ex- 
amples of vein-variation in different rocks. The basaltic caps 
and sills of that region overlie Animikie slates and argillytes, 
resting in turn upon taconytes and several hundred feet of 
cherts. These rocks rest upon the basal quartzyte overlying 
the Archean complex. The silver-veins are often from 8 to 10 
ft. wide in the slates, and carry native silver and argentite with 
small amoimts of blende, galena and pyrite, in gangue of 
quartz, barite, calcite, fluorite, etc. They are wide and pro- 
ductive in tb.e slates, but split up into narrow and barren seams 
in the overlying trap-rock, and are barren in the underlying 
cherts and Arcliean rocks. The Rabbit, Silver Mountain, 
Beaver and other mines have been noted producer.s. 
Kemp* describes the Silver Islet mine as a fissure-vein car- 
rying native silver, argentite, tetrahedrite, galena, blende and 
nickel and cobalt compounds in a calcite gangue. The vein 
is in fiags and shales of the Animikie series (Algonkian) and 
cuts a large trap-dike (gabbro), within i^'hich alone the vein 
is productive. 
* "Ore Deposits," 3d ed., 1900, p. 2.S3. 
