enter.] TREAD WELL ORE DEPOSITS, DOUGLAS ISLAND. 87 
ical date with, the Coast Range diorites strongly indicates the possi- 
ility of a great buried couche, or reservoir, of igneous rock under- 
ling the whole region. It is evident throughout the held that the 
eins were formed at a period subsequent to the invasion of the 
iorite, and they were probably formed long after intrusion had 
eased, but it is not a violent supposition to consider that the deep- 
Bated magma from which the masses now observed at the surface 
ad been given off remained in a molten condition for a very long 
me. 
A plausible hypothesis for the formation of the veins, based upon 
le foregoing ideas, is that the unknown forces which at various 
mes have caused general elevation throughout the region were trans- 
lated by this great residual magma to the overlying rocks. In 
dj listing themselves to the changed conditions of equilibrium, the 
3cks were fractured; then, as the deep-seated magma gradually cooled 
ad crystallized, water and gases expelled from it found their way into 
ie overlying rocks, and, searching out the easiest routes of travel 
long existing fractures, escaped to the surface. Undoubtedly waters 
f this origin might carry in solution all the elements which have been 
bserved in the veins, and they would deposit their mineral contents 
nder various conditions, such as decrease of dissolving power through 
iminishing pressure and temperature, precipitation through metaso- 
latic interchange with wall-rock materials, or precipitation due to 
lingling with solutions of some other derivation. 
SUMMARY. 
The large bodies of gold ore in the Treadwell mines are secondarily 
uneralized dioritic dikes lying between a hanging wall of greenstone 
nd a foot wall of black slate. The gold accompanies pyrite and other 
ilphides occurring both in reticulating seams of calcite and quartz 
id disseminated through the rock itself. 
Feldspar remaining from the original rock consists of oligoclase and 
licroperthite, but these have been largely replaced by albite through 
le metasomatic action of the vein-forming waters. 
The veinlets occur in two sets of fractures at right angles to each 
bher, which were probably produced by shearing stresses incident 
pon continental uplifting. Hot ascending solutions, possibly of mag- 
uitic origin, have been the cause of mineralization, and the evidence 
in favor of only one period of concentration. 
Secondary concentration of the metallic minerals being absent, there 
; no reason to anticipate any decrease in the per ton value of the ores 
3 greater depths are attained. 
