ORES OF GOLD 
44 
which ig telluride, can always be distinguished from the pre- 
Cambrian, and that as the local Kainozoic earth-movements 
Were tensional, the shearing and crushing of the Homestake 
Ore show that it must be pre-Cambrian. 
The Alaska Treadwell Mine at Jumeau, an island off the 
[28st of Alaska, is famous, like the Homestake Mine, for its 
long success in working low-grade material. The country is 
Carboniferous slate interbedded with altered lavas known 
a5 greenstones. Both rocks are intruded by albite-diorite 
dykes, Which have been shattered by earth-movements and 
enriched with gold-bearing quartz and sulphides, including 
Pyrite, stibnite, galena, blende, and molybdenite. Innumer- 
able sma veins ramify through the shattered diorite and form 
large bodies of ore, of the value of about 8s. a ton. Thanks 
to abundant water-power, the mines were worked on a great 
Scale at the cost of about 55. 6d. a ton. The difficulty in 
Mining was the maintenance of such large excavations. 
Though the veins were usually a few inches thick, they were 
So Crowded that the rock had to be removed for a width of 
200 feet, ang after the mine had been worked to the depth 
of over 1000 feet the timber pillars collapsed, the walls fell 
‘0 and the mine was flooded by the sea. 
The Porcupine Goldfield is of interest as the youngest 
¥ the great goldfields, having been discovered only in 1909. 
It is in northern Ontario (483° N., 81°W.), on the slope 
towards Hudson Bay. The field consists of Keewatin 
Pillow lavas, with well-preserved variolitic surfaces, and tuffs. 
This series was followed by the Timiskaming sediments in- 
cluding sandstones, in which the false-bedding is often well- 
Preserved slates ang conglomerates. Both these series have 
been ntruded by quartz-porphyry, which is also pre-Palzo- 
20IC. The relations of the rocks are well seen N. of the 
Dome Mine (Fig. 13) where the Timiskaming beds are gently 
foldeq into two anticlines and two synclines, and have at 
the base 4 thick conglomerate which rests on the lavas. 
The rocks at the mine are folded into an anticline with a 
Bentle dip to the N., but a very steep dip to a compressed 
SYacline on the southern side. Further S. is an extensive 
‘A. G. Burrows, ¢ : » 
Ontars, Dept, Mines, pe 15, Porouging Gold Area,” 337d Ann. Rep