spencer] TREADWELL ORE DEPOSITS, DOUGLAS ISLAND. 75 
The primary feldspars of the magma were albite-oligoclase, occurring 
in phenocrysts now always clouded by decomposition products, and 
microperthite with some pure albite, forming a granular groundmass 
of distinctly later crystallization. The composition of the phenocrysts 
is inferred in general from the presence of epidote as one of the min- 
erals formed by the alteration of the feldspars, but this has been 
checked by the optical characteristics of relatively fresh material occur- 
ring in several specimens. The secondary feldspar is always albite, 
and is usually quite free from decomposition, and when it occurs in 
sufficient amounts it gives the rock a very fresh appearance. It seems 
to have been formed mainly at the expense of the original microper- 
thite, which it replaces in part. 
Quartz seems not to have been an original mineral in the albite- 
diorite, and it is never observed in the body of the rock associated with 
the secondary albite, but is confined to the veinlets which intersect the 
dikes. Calcite is common both in the veins and distributed through 
the rock itself along with the albite of the second generation. 
Original accessory minerals noted are apatite, titanite, rutile, and 
magnetite. The secondary minerals which have been noted are uralite 
(secondary hornblende), green mica, chlorite, epidote, zoisite, calcite, 
quartz, sericite, rutile, pyrite, pyrrhotite, and stibnite, with other sul- 
phides occurring exceptionally. Some of the magnetite seems also 
to have originated from the breaking up of former iron-bearing min- 
erals, and where it surrounds cubes of pyrite it has apparentl} T been 
deposited from the mineral solutions. 
In the vicinity of the mines dikes of albite-diorite in the black 
slates are distributed throughout a zone about 3,000 feet in width, 
1 extending along the strike for 3 miles. Only bodies near the hanging 
wall of this zone have been mined up to the present time, though 
several others are strongly mineralized. The dimensions of the dif- 
ferent dikes are extremely variable, the larger ones having a maximum 
i observed width of over 200 feet in surface exposure and in the mine 
workings. From this all sizes occur down to the width of one's hand, 
and toward the ends of the intrusive area only small dikes occur, as 
may be observed along the bed of Bullion Creek. 
Outside of the ground which has been worked, the details of the 
various diorite masses are unknown, but their general distribution is 
shown upon the geologic map, and the generalized cross section through 
the workings of the Treadwell mine indicates the relative number and 
size of the dikes which outcrop (figs. 1 and 2). Undoubtedly a still 
larger number, principally of small dikes, are hidden by gravel beds 
and by the deep mat of decaying vegetation which covers much of the 
ground. 
In many cases — and this is particularly to be noted in the dikes which 
have been mined — the individual intrusions are made up of a series of 
