Intincncc of Coiintry Rock. — Weed. 
I8l 
Pontgibaiid, France. — Here the silver-lead veins occur 
along fractures within granulyte dikes, and on the line of con- 
tact with the gneiss country. 
"When the dike diminishes in size the vein decreases in width; 
when the vein penetrates the gneiss the ore disappears. The best ore 
is associated with the kaolinization of the feldspar of the granulyte; 
when the latter becomes hard, unaltered in depth, the ore pinches out."*^ 
Rico, Colorado. — Rickard also mentions the veins of rich 
gold- and silver-ores at Newman Hill. Rico, Colo., as "notice- 
ably affected by the character of their rock-walls." There is a 
marked change in passing from limestone into sandstone ; and 
generally tlie veins are richest in the darker-colored sedi- 
mentary rocks. The interdependence between country-rock 
and ore is briefly discussed by Rickard, who adopts Cotta's 
explanation that the physical textttre and chemical composition 
of the cottntry-rock aifect the deposition of ore. and declares 
that It was undoubtedly the carbonaceotts matter of the rock 
at Rico which acted as precipitant. This example dififers, 
_ therefore, from that of Pontgibaud. where feldspar has been 
replaced by silver-bearing galena. 
NeiJiart, Montana.- — At this locality the veins show remark- 
able variation in richness, corresponding to differences in the 
FiQ. 7 
Gray gneiss; pay ore. 
"^ Black gneiss; no ore. 
Ked gneiss; good ore, 
Dark-gray schist no ore. 
Diagram (Plan) Showing Rocks Traversed by the Neihart Veins, and 
Relation of Pay-Ore to Country-Rock. (The veins are 
indicated by simple jiarallel lines.) 
wall-rock. ]^lr. Robert H. Raymond, the former manager of 
the Diamond R. properties, found that the veins were barren 
• T. A. Rickard, "Vein Walls," Trans., xxvi , 200 (1897.; 
