94 ^Ji^ Amenca7i Geologist. Febmary, loots 
believe its auriferous contents to be clue to impregnation from 
neighboring veins. Two other systems of gold-bearing dikes 
are not so easily disposed of. The white quartz-porphyryte of 
the Upper Coffee creek district appears to carry manganese 
oxide and free gold wherever it has been tested, and I am in- 
clined to believe the precious metal is present in this as a pri- 
mary constituent. Probably concentration of the gold has oc- 
curred on certain lines within the dike and along its walls, but 
mines opened on this system of dikes may be depended on with, 
reasonable certainty to be of a permanent nature. The richest 
deposits in connection with the quartz-porphyryte are found in 
faulted belts along its walls. 
In the Tamarack and Lower Cofifee creek districts, the 
richest veins contain, between the walls of the crushed belts, 
narrow, branching dikes of dioryte-porphyryte of the later 
system. From the fact that they have not suffered any great 
crushing, I infer that they are of the same age as the mineral 
veins. The same faulting which permitted the ascent of the 
mineral-bearing waters, admitted of their injection far up in 
the strata. They carry a little gold, but are too narrow to have 
supplied the precious metal to neighboring portions of the 
veins. Whether we consider the gold in this dioryte-porphy- 
ryte as primary in origin or not, we must acknowledge that the 
auiiferous contents of the veins, if not derived from the meta- 
morphic rock or grano-dioryte of the walls, must have ascend- 
ed from a considerable depth. 
6. The close association of valuable mines with the contact 
between porphyry dikes and the country-rock is a matter of 
common observation in western mining districts. Except in a 
few cases, like that of the quartz-porphyryte of the Upper Cof- 
fee creek district, there is probably rather an accidental connec- 
tion between them. When the intrusives cool, they contract, 
thus tending to form a more or less continuous open space 
along one or both of their walls, which may descend to great 
depth, constituting favorite sites for the ascent of the mineral- 
bearing solutions. Again, the contact of a dike with the coun- 
try-rock is a line of weakness in the strata along which faulting 
is more likely to occur than on any other. Hence, the ob- 
served association of many of the richest veins with the dikes is 
no proof of the derivation of the gold from the intrusives. 
