fencer.] TREADWELL ORE DEPOSITS, DOUGLAS ISLAND. 73 
ing wall, and the latter grades off into the massive rock. This rela- 
tion suggests that the localty developed schistosity of the greenstone 
existed before the intrusion of the diorite dikes or was produced at 
the time of their invasion, and in either case the greenstone must be 
the older rock. More definite evidence in the same direction was 
noted in an old stope above the 220-foot level in the Treadwell mine. 
Here the main mass of diorite lies below all of the greenstone, but the 
iatter is somewhat schistose, and a narrow offshoot from the diorite 
2uts across this secondary structure for a distance of about 8 feet, 
ind then follows the schistosity parallel with the wall of the large 
Dre body. 
Without the above proof that the diorite is intrusive in the green- 
stone, several general considerations would lead to the probability of 
:his relation. In the region at large the dioritic rocks invariably cut 
;he bedded greenstones, and in Sheep Creek they are even later than 
he gabbro dikes which follow the structure of the inclosing rocks 
ipproximately. None of the basic intrusives which are evidently later 
han the Coast Range diorites show any tendency to follow the struc- 
ural trend of the region, but, like the small basalt dikes in the Tread- 
veil mine, they characteristically hold to transverse courses. The way 
n which the greenstone limits the zone of diorite dikes, and the marked 
oberence of individual dikes to its lower surface, both point to the 
langing-wall stratum as a controlling feature in the distribution of the 
liorite, and therefore suggest its earlier existence. The probability 
)f this connection is well brought out by the map and cross section. 
\.gain, if the attitudes of the diorite dikes and the greenstone in 
eference to the slate country are compared, it is found that the 
iorite shows all the ordinary structural characteristics of intrusions, 
Ivhile the greenstone exhibits no features which necessarily require an 
titrusive origin. The diorite bodies change in shape from place to 
1>lace, branch irregularly, crosscut the stratification locally, and include. 
lasses of slate. The greenstone is a single layer or bed which con- 
inues along the same horizon for at least 6 miles and shows but slight 
ariations in thickness; it does not crosscut the slates, so far as 
bserved, and it contains no slate inclusions. Under the circumstances 
: is strongly believed that the greenstone is not intrusive, but that it 
riginated as a lava flow similar to many others in the same general 
eries of alternating sediments and igneous rocks, while the diorite 
ras intruded at a much later date. 
BLACK SLATE. 
The black slates, which constitute the main country rock near the 
readwell mines, belong to the third subzone of the slate-greenstone 
and already described. Together with the hanging-wall greenstone 
ey constitute all of this subzone which appears on the southern half 
