brooks.] PLACER GOLD MINING IN ALASKA. 43 
by the sorting action of water. It is not uncommon to hear Alaskan 
prospectors speak of the "mother lode," as if the gold had all been 
derived from one lode or zone of mineralization. Of this there is no 
evidence whatever. In considering the question of quartz veins in 
the placer fields, it should be remembered that the dense coating of 
moss makes bed-rock prospecting difficult and uncertain. 
The auriferous deposits from which the placer gold is derived occur 
in metamorphic rocks of various kinds. They include schists of vari- 
ous types, phyllites, limestones, quartzites, and altered igneous rocks. 
Such metamorphic terranes find a wide development in Alaska, and 
probably occur in a number of different horizons. The study of the 
geology of Alaska has not progressed far enough to permit of correla- 
tions, or of definite statement in regard to the age of the metamorphic 
terranes or their structural relations. The mineralized metamorphic 
beds of southeastern Alaska are probably Mesozoic and older. Those 
of the Yukon are chiefly, if not entirely, pre-Carboniferous, and those 
of the Seward Peninsula are chiefly Paleozoic. Within the zone which 
\ has been designated as the one in which gold placers have been found, 
there are many large areas of these metamorphic rocks. These form 
belts which are not by any means continuous, as they are interrupted 
b ; areas of younger Mesozoic and Tertiary terranes. It has also been 
shown that they probably belong to widely different horizons. Broadly 
speaking, the mineral-bearing horizons of southeastern Alaska can be 
placed in one group, and those of the Yukon Basin and of the Nome 
\ region in another. It will remain for future studies to determine the 
relation between these two belts. 
The age of intrusion of the mineral-bearing solutions is largely an 
unsolved problem. In the coastal belt of southeastern Alaska the 
mineralization took place probably in Mesozoic time, while in the 
Yukon region it was probably considerably earlier. The studies thus 
far made indicate that the mineralization accompanied disturbances 
of the strata, either by deformation or by igneous intrusions, or both, 
which were rather local in their effect. They seem to be closeby 
affiliated to igneous rocks which are everywhere found in the regions 
of mineralization. 
The studies of the alluvial gold deposits of Alaska have shown that 
'mode of formation and concentration are the determining factors 
of the richness of the placer deposits. The writer has elsewhere a 
emphasized this fact in regard to Nome placers, and more recent 
observations convince him that it is also applicable to the gold deposits 
of the Yukon. In the simplest form of placers the gold is washed from 
the parent rock and concentrated in the beds of the streams, mingled 
with other detrital material Such placers have been exploited in 
many localities and have been found to be important gold producers. 
a Reconnaissance ot the Cape Nome and Norton Bay Regions, U. S. Geological Survey, 1901, pp. 
L44-151. 
