72 ALASKAN MINERAL RESOURCES IN 1904. [bull. 259. 
GREENSTONE. 
The hanging-wall greenstone forms a prominent bed or stratum 
about 300 feet in thickness where measured in the mines, but varying 
somewhat from this figure in different parts of its outcrop. So far 
as can be determined, it follows the structure of the slates, striking 
with them from southeast to northwest, and dipping northeast toward 
the near-by channel, beneath which it has been followed to a depth 
of about 900 feet in the lowest workings. The outcrop is practically 
continuous for 4 miles northward from where the greenstone first 
appears on the shore of Douglas Island. Then the bed thins out and 
is wanting for a few hundred feet, but it soon reappears and may be 
followed for an additional 2 miles, until it is lost beneath a heavy 
covering of vegetation. 
As a rule, the rock is greatly altered, and in places it is even schis- 
tose or slaty, but portions are sufficiently unchanged to indicate the 
original composition and structure. In the vicinity of the Ready Bul- 
lion mine the rock is granular, consisting mainly of coarsely crystal^ 
lized hornblende, though it contains a great deal of magnetite and 
some pyrite. A specimen from the Mexican workings, which might 
be called andesite, contains porphyritic crystals of plagioclase and 
augite in a decomposed groundmass, which seems to have consisted 
largely of small prismatic feldspar crystals. The secondary minerals 
are chlorite, epidote, serpentine, and calcite. Beyond the workings 
toward the northwest the greenstone is a fine-grained diabase. 
The greenstone was called gabbro by Becker, who regarded it as 
later than the rock of the ore bodies, but there is now sufficient evi- 
dence to establish the opposite age relation, and reasons exist for 
doubting its intrusive nature. The inclusions of light-colored rock 
fragments in the greenstone, which form the basis of Becker's conclu- 
sions, are represented in his collection by a specimen and a thin sec- 
tion, showing a distinctly outlined fragment of grayish granitoid rock 
inclosed in greenstone; but the diagnostic value of this occurrence is 
open to doubt, since at several points in the region pebbles and frag- 
ments of similar material occur in the volcanic greenstone breccias at 
different horizons in the series of interbedded slates and greenstones, 
showing the existence of an available source of granitoid material 
prior to the deposition of the slates and the outpouring of the con- 
temporaneous lavas. 
In the open pits of the Seven- Hundred Foot and Mexican mines 
the exposed lower part of the greenstone bed is very schistose, and 
this slaty rock forms both walls of the ore body. Between the ore 
and the black slate usually forming the foot wall there is a plate or 
layer of chloritic schist of somewhat variable thickness, evidently 
identical with the schistose or slaty greenstone of the immediate hang- 
