GENERAL GEOLOGY 
31 
are black-bordered. There is a glassy groundmass, full of 
feldspar microlites. The rock is closely like the lighter 
lava described by Dr. Geo. P. Merrill 1 from the bombs of 
the neighboring island Grewingk, or New Bogoslof, which 
protruded itself from the sea in 1883. It has a finer, 
uniform, dusty groundmass, and the hornblende and pyrox¬ 
ene crystals are smaller, equal in number, and more dis¬ 
tantly scattered. The section of one feldspar crystal is cut 
parallel to the orthopinacoid (100), the long sides are 
formed by the traces of the domes (xoi) and the ends by 
those of (021), and a positive bisectrix appears almost cen¬ 
tral with the axial plane about parallel to one of the (021) 
faces. The extinction is +41 0 , showing that the central 
plate is an anorthite cut about at right angles to axis a. 
Rotated 6o°, so that the axis b takes the place of a in 
the line of sight, the extinction is 52 0 . The first band out¬ 
side the central plate extinguishes in a quite broad band 
from +31 0 to + 33 0 , or about at labradorite (abj an^. 
Within this band is a narrow thread which extinguishes at 
41 °, and is thus a nearly pure anorthite, like the centre; 
and directly inside this is a narrow band which extin¬ 
guishes at + 22 0 , and so is near (abjani). The next broad 
band has three subordinate flutings, but the extinction pro¬ 
gresses outwardly with much regularity from + 28° to o°, 
or from labradorite (abj an x ) to oligoclase (ab 4 an^. The 
outer band is but slightly fluted, and extinguishes from 
+ 20 0 to — x i°, or from near labradorite (ab x an 1 ) to a nearly 
pure albite (a pure albite would demand an extinction of 
— 15 0 in this position). 
PRIBILOF ISLANDS 
At the Pribilof Islands we landed on St. Paul near the 
interesting Black Bluff, a symmetrical remnant of a cinder 
cone mostly dissected away by erosion of the waves. Mr. 
1 Proc. U. S. National Museum, vol. viii, 31. 1885. 
