The Aleutian Islands region (p1. 1, fig. 2) includes the islands
west of Unimak and is considered as one district. It consists of
an archipelago surmounting a submarine ridge 1,400 miles long and
20 to 60 miles wide that rises 12,000 feet above the sea floor.
An arcuate line of about 40 volcanoes, 21 of which have been active
in historic time (Coats, 1950), rises as much as 6,975 feet above
The region is underlain by Cenozoic basalt and andesite lava flows,
tuffs, and clastic rocks and by mafic to felsic dikes, sills, and
stocks. Recent work by Scholl and others (1969) has shown that rocks
formerly thought to be Paleozoic(?) in age (Coats, 1956, p. 48—49)
are Eocene. This region is geologically the youngest in Alaska,
as all others are at least partly underlain by rocks as old as Mesozoic.
The only lodes known in the Aleutian Islands region are gold deposits
(some of which may have been productive) and occurrences of metallic
sulfide minerals on Unalaska and neighboring Amaknak and Sedanka
Islands (Berg and Cobb, 1967, P. 7—8, fig. 2). Capps (1934,
p. 149) reported rumors of copper on Salt Island, off the north
shore of Atka, and speculated that the copper might have been derived
from an amygdaloidal lava flow.
The only placer occurrence reported in the Aleutian Islands is a
few grains of gold in tributaries of the Makushin River (fig. 2)
on Unalaska Island. The occurrence is near a conspicuous gossan
of severely altered and pyritized volcanic rocks near a small body