42 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
mines of Alaska which have thus far been opened up fall within a 
zone having a maximum width of probably 200 to 300 miles, stretch- 
ing northwest from the southern Pacific coast, crossing the Arctic 
Circle, and bending westward to the shores of Bering Strait. It is 
not intended to imply that this zone in its entirety is a gold producer; 
such is far from being the case. This broad belt is simpty drawn 
attention to as having, up to the present time, been the locus of the 
placers of commercial importance. The factors which have deter- 
mined the formation of workable placers are frequently so local in 
their effect that the distribution of the placers is very irregular. 
The field studies lead to the conclusion that the source of the gold 
lies, for the most part, in small quartz veins and stringers which are 
disseminated in metamorphic rocks. Gold also occurs in these rocks 
in the mineralized zone, where there is little if any gangue mineral 
present. Iron pyrite is the commonest mineral found in association 
with the gold in the parent rock. The few observations made indi- 
cate that the gold occurs both free and combined with pyrite. Quartz 
is a common gangue mineral, associated with some calcite. Galena 
is frequently associated with the gold-bearing quartz veins, and chal- 
copyrite and arsenopyrite have also been found. This list of minerals 
will undoubtedly be much extended when closer studies have been 
made. 
The studies of the placer fields of Alaska lead to the conclusion that 
the gold in nearly every case lias not traveled far, and can usually be 
traced to a local source. In the gulch and creek placers it can usu- 
ally be traced to a source within basins which they drain. The excep- 
tion is where a change of drainage may have introduced material 
derived from regions outside the creek basin. In nearly all parts of 
Alaska the placer gold owes its present position entirely to the erosion 
of the bed rock in which it was formerly disseminated, and to the sort- 
ing action of water and gravity, which has brought about its pres- 
ent concentrated form. This elementary principle is here emphasized 
because it is not uncommon to find, even among well-informed men, a 
tendency to entirely ignore the very simple facts, and to regard placers 
as the result of glacial action, or as having had a still more cataclysmic 
origin. As a matter of fact, all of the placers of Alaska, except a few 
near the southern coast, are outside of the limit of former glacial 
activity. 
As has been stated, the gold of the placers has its source in small 
veins and stringers in the bed rock or was disseminated in mineralized 
zones. The facts now obtainable indicate that the outlook for future 
quartz mining in the placer fields of the interior of Alaska is not hope- 
ful. While it is by no means impossible that larger gold-bearing 
veins carrying commercial values may be found, it seems probable 
that most of the placer gold has been freed from bed rock, where it 
was more or less widely disseminated, and subsequently concentrated 
