Note on Hypersthene-Andesite from Alaska. 157 
May 18th, 1897. 
Mt. Edgcumbe is a volcanic cone situated on Krusov island, 13 miles 
west of Sitka, Alaska. It has quite regular outlines and its altitude is 
given in the U. S. Coast and Geodetic Survey maps as 2855 feet. Its 
lower slopes are thickly wooded but the upper part of the mountain is 
quite bare. The rim of the crater is a broad oval of something more 
than a thousand yards along its longest diameter. There is a difference 
of about 400 feet in the hight of the lowest part of the rim (on the south- 
ern side) and the highest point (on the northwestern side). The bottom 
of the crater is also oval with a slightly sloping floor about 230 feet be- 
low the lowest part of the rim. Sections of former lava flows are visi- 
ble on the steepest parts of the inner wall of the crater. Lava flows ex- 
posed on the southeastern shore of the island still preserve in fjlacee 
their original surface. Everything shows that the volcano has not long 
been extinct, but it probably has not been in eruption since the discov- 
ery of Alaska by the Russians, 150 years ago. 
Mist was driving over the mountain when I was on the summit, and 
I did not have a good view of the region surrounding the mountain, 
but Prof. Wm. Libbey has given a short description of this region in 
the Bull. Am. Geog. Soc. 1889, pp. 279-288. 
All the specimens I sent were gathered in the crater or on the rim in 
September, 1892. 
Harry Fielding Reid. 
The specimens are of a (juite normal hyperstheiie-tiugite- 
andesite, the interest attaching to them being on account of 
their geographical location and of their similarity to the vol- 
canics to the southward along the same volcanic belt. They 
range from compact dark gra}' rocks dc>tted with small feld- 
spar and pyroxene phenocrysts to a nearl}' black pitclistone 
and to a light colored, frothy pumice. 
Jfineralogical composition. Plagioclase, hypersthene, au- 
gite and magnetite, in order of abundance, make up the slides. 
Two generations are invariablj^ present. In two cases aggre- 
gates similar to those which result from the corrosion of horn- 
blende, were noted, but no trace of the original mineral 
remains and the boundaries are indefinite. 
The plagioclase phenocrysts are stout prisms bounded by 
many planes, often forming complicated intergrowths, with 
pronounced zonal structure and dearth of twinning. Extinc- 
tion angles averaging 28° on opposite sides of the trimming 
plane shf)W them to belong to the bytownite-anorthite series. 
They abound in inclusions of glass, pyroxene and magnetite. 
The zonal structure is of both types, in some cases a regular 
