88 MicrosG02ncal Character of the Ore. — Adams. 
acter to that here described remain to be discovered. With 
the facts developed in connection with this mine in view it 
would appear to be well worth while to subject to examination 
and assay all pyritous granitoid rocks in contact with or pen- 
etrating the sedimentary formations, and in particular those 
which may be found to traverse the slaty argillites of the for- 
mation above referred to as the Vancouver series. 
ON THE MICROSCOPICAL CHARACTER OF THE ORE OF 
THE TREADWELL MINE, ALASKA. 
By Frank D. Adams, of the Geological Survey of Canada. 
(Read before the Royal Society of Canada, May 8th, 1889.) 
The material employed in this examination consisted of several 
small specimens kindly placed at my disposal by Dr. Geo. M. Dawson 
who collected them at the Treadwell mine in 1887, together with two 
collected by Mr. R. G. McConnell, of this survey, who visited the mine 
when returning from the Yukon country last autumn. 
The ore is a more or less altered granite, rather coarse in grain and 
of a light grey color. As mentioned by Dr. Dawson in the previous 
paper it encloses "kernels" often greenish in color and distinctly 
granitoid in appearance, having a diameter of from six inches to sev- 
eral feet. These are of the same grain as the rest of the mass, but are 
harder and less evidently decomposed, and pass rather sharply but 
imperceptibly into the ordinary grey granite. As these represent the 
granite in its least altered form they will be described first. 
The Kernels. — One of the hand specimens shows a portion of one of 
these"/;<'rne^s" which is seen to differ from the ordinary granite in two 
particulars : 1st, in being light reddish in color instead of grey ; 2d, in be- 
ing free from quartz-veins and holding but little pyrite. When a thin 
section is held against a dark background it is seen to be made up of 
numerous rather large translucent crystals or individuals closelj' 
packed together, but separated by narrow, transparent, intermediate 
lines. Under the microscope these translucent crystals are seen to be 
feldspar a good deal decomposed (which accounts for the opacity) while 
the intervening spaces are found to be in part grains of quartz or of 
broken feldspar and in part the edges of feldspar crystals, which are 
often much freer from decomposition jiroducts than their central 
portions. 
Most of the feldspar is untwinned and is referred to orthoclase. 
A much smaller amount, however, shows polysynthetic twinning, in a 
few cases two sets crossing at right angles, and is therefore plagioclase. 
In one of the sections a few large grains showing perthitic intergrowths 
were seen. Both feldspars often possess a marked zonal structure, 
caused or accentuated by the accumulation of decomposition products 
along certain concentric lines. Although many of the feldspar indi- 
viduals extinguish simultaneously over their whole extent many 
