482 
Proceedings of the Royal Society 
clase, and the most characteristic of this ash, although represented 
very rarely, are in the form of rhombic tables, extremely thin, and 
covered with a fine lacework of vitreous matter. We know that 
the crystals described by Penck * in a great number of lapilli 
and of volcanic ashes, upon the nature of which doubts have been 
expressed, belong incontestibly to the plagioclases, and represent an 
isomorphic mixture analagous to that of bytownite. It is to Mr 
Max Schuster t that we owe this specific determination. Having 
found in numerous sediments of the Pacific these same crystals in 
the form of rhombic tables, and possessing preparations which would 
be of great interest to him in his remarkable optical studies on the 
felspars, we submitted them to this ingenious mineralogist in order to 
confirm our determination. We believe it will be interesting to give 
a resume here of the results of the observations of Mr Schuster, 
which are perfectly applicable to the characteristic crystals of felspar 
from Krakatoa, as well as to those which we have discovered in a 
great number of deep sea soundings. 
This plagioclase occurs for the most part in flat tabular crystals 
with the clinopinakoid especially developed. Individuals of the 
columnar type, elongated in the direction of the edge P/M, are rare. 
These tabular crystals consist essentially of a combination of the 
clinopinakoid with P and x, more rarely with P, u, and y, and 
occasionally x and y appear together. In the first case the crystals 
have the form of a rhomb, in the second case they are elongated 
through the predominance of either x or P. The dimensions of 
those crystals which were examined and measured, lie between the 
line 0-61 mm. broad and 1 mm. long as maximum, and 0’015 mm. 
broad and 0'042 mm. long as minimum. The extinction of the 
plagioclase is negative. Its value was found to vary between 22° 
and 32° on the clinopinakoid, and between 8° and 16° on the 
basal plane. The average values of many measurements made on 
good crystals are as follows: — 24° 12', 25° 6', and 29° 6' on the 
clinopinakoid, 10° 42' on the one side, and 10° 18' on the other 
side of the twinning line, as this is shown on the basal plane. 
* Penck, “Studien iiber lockere vulkanische Auswiirflinge,” Zeitsclir. d. 
dcutsch. geol. Gesellscli., 1878. 
4 Schuster, “ Bemerkungen zu E. Mallard’s Abhandlung sur risomorpliisme 
des feldspaths triclinigues, &c.,” Min. petr. Mittli., v. 1882, p. 194. 
