20 S. f. EMMONS 
level mines have been opened; 2. ¢., those whose surface lines do 
not include the outcrop, but which like the Tamarack and other 
conglomerate mines at Lake Superior, first reach the payable 
beds at considerable depths. How far from the outcrop toward 
the middle of the basin it may be profitable to open deep level 
mines, depends upon three factors: 
1. The angle at which the beds descend, or the depth at 
which they will be found under a given point on the surface. 
2. The vertical depth at which the difficulties of mining will 
neutralize the profit. 
3. [he extent in depth, or the distance from the outcrop to 
which the conglomerate beds will continue to be payable. 
If the gold is entirely placer gold, that is if it has all been 
brought into the beds mechanically, by the action of sea waves 
—there has been no suggestion of old river channels—it is evi- 
dent that there must be a limit to the distance from the shore to 
which so heavy a metal could have been transported. 
That wave action may concentrate the gold in beach gravels 
sufficiently to constitute workable placers, has been proved on 
the coasts of California, Oregon and Alaska, though practically 
such placers have not yielded much profit to those working them, 
because the pay streaks are constantly shifted by storms and 
ocean currents. 
The best authenticated instance of a fossil placer on an old 
shore line (not a river bed) known to the writer, is the conglom- 
erate at the base of the Potsdam sandstone, near Deadwood in 
the Black Hills. According to W. B. Devereux,’ who has given 
the best description of these deposits, the conglomerate which is 
rich enough to be profitably mined occupies a narrow belt, not 
over one and one-half miles wide, in the immediate vicinity of the 
Great Homeslates group of deposits in the underlying crystalline 
schists, from the degradation of which it is evidently derived. 
The gold inthe Potsdam quartzite of Bald Mountainand other dis- 
tricts of the Black Hills, however, is found, not in rounded pellets 
and flattened flakes, but finely divided and, when visible, is in the 
tTrans. Amer. Inst. Mg. Engrs., Vol. XVII, p- 572. 
