GENERAL GEOLOGY 3! 



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 l 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 (101) 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 +4I , showing that the central 

 plate is an anorthite cut about at right angles to axis a. 

 Rotated 60, so that the axis b takes the place of a in 

 the line of sight, the extinction is 52. The first band out- 

 side the central plate extinguishes in a quite broad band 

 from +31 to +33, or about at labradorite (ab t anj. 

 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, and so is near (abjanj. The next broad 

 band has three subordinate flutings, but the extinction pro- 

 gresses outwardly with much regularity from + 28 to o, 

 or from labradorite (ab A anj to oligoclase (ab 4 anj. The 

 outer band is but slightly fluted, and extinguishes from 

 + 20 to 1 1, or from near labradorite (ab x an 1 ) to a nearly 

 pure albite (a pure albite would demand an extinction of 

 15 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. vui, 31. 1885. 



