spencer] TREADWELL ORE DEPOSITS, DOUGLAS ISLAND. 
71 
hanging wall strikes the shore of the island about 1 mile below the 
Ready Bullion mine, at first running inland and then back to a point 
below high water just below where the southernmost body of diorite is 
exposed in the open pits of the Ready Bullion mine. Reappearing 
within a few hundred feet, it bends sharply and is next exposed in the 
southeast pits of the Mexican mine. From this point it is traceable 
in a nearly straight line through the Seven Hundred Foot and Tread- 
well workings and for a distance of several miles beyond. 
In the vicinity of the mines there are no dikes of diorite on the 
channel side of the greenstone, but about 1 mile to the northwest two 
croppings have been noted, and Juneau Island, in Gastineau Channel 
about 2,000 feet from the foot wall, is composed of similar rock, which 
is somewhat impregnated with pyrite. 
Fig. 1. — Sketch map showing geology near Tread well mines. 
Besides the mineralization of the igneous dikes, the black slates of 
the same general belt on both sides of the greenstone band contain 
occasional veins and systems of quartz stringers following the struc- 
ture. Veining of this sort has been particularly noted along the foot 
wall of the Treadwell greenstone for a distance of several miles beyond 
the mines. Assays of about $6 per ton in value have been obtained in 
some places, but there has been no systematic attempt to develop these 
stringer leads, and their value is doubtful. 
The rocks occurring in and near the mines, which will now be 
described in greater detail, are the following: The greenstone hanging 
wall; the slate country rock, inclosing both greenstone and ore bodies; 
the dikes and lenticular masses of diorite, some of which constitute 
the ore; and a few small dikes of basalt. 
