spencer] TREADWELL ORE DEPOSITS, DOUGLAS ISLAND. 77 
regarded as the once deep-seated portions of intrusions which probably 
had actual surface exit. In the underground workings the blind end- 
ings of certain of the dikes show that some of them do not extend even 
to the present surface. How much farther the larger ones may have 
penetrated the slates now removed by erosion can not be estimated. 
BASALT DIKES. 
In several places in the mine workings there are basalt dikes, which 
cut all the other rocks. They are narrow, usually from a few inches 
up to 3 feet in width, and have sharply defined walls. Locally, 
the dikes occur in pairs, and in several places are seen to divide, par- 
ticularly when they occur in zones of sheeted rock. The fissures in 
which the}^ occur are transverse to the strike of the rocks and trend 
from N. 10° W. to about north and south, true meridian, with a rather 
steep dip toward the west. As a rule, they are not mineralized to any 
important extent, though a small amount of pyrite sometimes appears, 
and occasionally they contain a considerable amount of this mineral. 
In several places veinlets of calcite occur along the selvage, but these 
are readily determinable as of later orign than the greater part of the 
quartz and calcite which form a reticulation throughout the mass of 
the ore material. 
THE ORES. 
GENERAL DESCRIPTION. 
The ore of the albite-diorite dikes consists mainly of rock impreg- 
nated with sulphides, principally pyrite, in part shattered and filled 
by reticulating veins of calcite and quartz, which also carry sulphides. 
The ore-bearing dikes are considerably mineralized throughout, and 
often the whole mass can be mined. Locally, however, the values are 
too low to pay for extraction, and portions of the rock must be left. 
Three sorts of ore are recognized by the miners, "quartz," " brown 
ore," and " mixed ore." The so-called quartz ore, which constitutes 
the bulk of the workable material, is essentially mineralized diorite, 
but it usually contains calcite and quartz, the calcite disseminated or 
in veins, the quartz only in veins. As a rule, it is white or light gray, 
out in many places it has a greenish cast. The brown ore is derived 
from a comparatively small amount of productive mineralization 
occurring in the walls or in the narrow horses of slate, where the presence 
)f gold-bearing sulphides is commonly recognized by a brown color, 
which leads to the popular designation of this ore. The brown mate- 
rial grades into the ordinary black slate, and its color is apparently due 
X) decarbonization of the carbonaceous rock by percolating sulphide 
solutions. Impregnation of the slate is by no means general, and 
there it occurs it seldoms extends for more than 2 or 3 feet from the_ 
alls of the main ore mass. The mixed ore, which is more abundant 
