CIRCLE PRECINCT. 189 
a number of steam bucket hoists. Winter work is now usually done 
kvith the aid of steam thawers. 
The placers here to be described fall into two groups that differ 
both geographically and geologically — (1) placers lying within the 
[Birch Creek basin and (2) those along streams which discharge 
directly into the Yukon. The gold of the first group is derived 
from mica-schist and quartz-schist bed rock; that of the second group 
is, in part at least, derived from a conglomerate, where it is of sec- 
ondary origin. 
BIRCH CREEK BASIN. 
GENERAL GEOLOGIC FEATURES. 
The known auriferous portion of the Birch Creek basin embraces 
primarily those streams which head in an irregular northwest- 
southeast trending ridge, of which Mastodon (4,500 feet) and Por- 
cupine (4,900 feet) domes form the highest summits. The radial 
arrangement of the gold-bearing streams from this watershed is a 
striking feature and is suggestive of the location of a zone of 
mineralization. 
Schistose quartzite and mica schist form the prevailing bed rock 
throughout the area. Locally these rocks are found to be f eld- 
spathic, and these phases may be altered intrusives, but for the most 
part the formations appear to be of sedimentary origin. The rocks 
are closely folded and much sheared, and the prevailing strikes are 
east and west. Granite intrusives are not uncommon. Notably on 
Deadwood, Mammoth, and Miller creeks there are considerable areas 
of this rock. The central parts of the intrusives appear to be mass- 
ive, but along some of their margins the writer observed evidence 
of deformation. Whether this is generally true he was unable to 
determine. Prindle has described some diabase dikes which occur 
in this region, but none came under the observation of the writer. 
A general wide distribution of vein quartz is attested both by the 
bed-rock exposures and by the character of the fluvial deposits. 
This quartz is very frequently found to be iron stained, and one 
naturally turns to it to seek a source of the placer gold. There is 
but little direct evidence on this point. The presence of pyrite- 
bearing vein quartz in the auriferous alluvium is a characteristic 
feature of these deposits. On Eagle Creek a 4-foot gold-bearing 
quartz vein is said to have been encountered in the drift mining, 
but the writer did not see the exposure, as the drift had caved in. 
A specimen of the quartz showed it to be iron stained and broken 
by thin seams of gold. The gold of the adjacent placer was angular 
and carried much quartz. A mineralized fracture zone about 8 
inches in width has been found on the upper part of Deadwood 
Creek. Within this zone the schist is permeated by stringer veins 
