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, 195Q), rises as much as 6,975 feet above sea level.

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 young¬est 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 of granodiorite.