GENERAL MEETING. 33 
Mr. J. S. DILLER made a communication on 
THE VOLCANIC SAND WHICH FELL AT UNALASHKA OCTOBER 20, 
1888, AND SOME CONSIDERATIONS CONCERNING ITS 
COMPOSITION. 
[Abstract. ] 
The sand is composed chiefly of crystal fragments of feldspar, 
augite, hornblende, and magnetite, with a considerable proportion 
of microlitic groundmass and a very few splinters of volcanic glass. 
Its mineralogical composition is that of a hornblende andesite; but 
the chemical analysis by Mr. Chatard shows it to contain only 
52.48 per cent. of silica,—which is much more basic than the average 
for that group. The character of the minerals, as well as the gen- 
eral composition of the sand, indicated so clearly that the crater 
from which it must have issued was erupting hornblende-andesite, 
that I was led to seek an explanation for its paucity in silica. 
With this purpose in view, a number of volcanic sands and dusts 
from various parts of the world were examined and compared with 
the lavas to which they belong. First and most important among 
these is a sand from Shastina, a crater named by Captain Dutton, 
upon the northwestern flank of Mt. Shasta, in northern California. 
This sand, like that from Unalashka, is composed chiefly of crystal 
fragments of feldspar, augite, hornblende, and magnetite, with 
fragments of microlitic groundmass. Besides these, there are many 
pieces of hypersthene crystals and pumiceous glass. The sand con- 
tains 60.92 per cent. of silica, while the hornblende-andesite lava 
(rich in hypersthene) of Shastina, to which the sand belongs, con- 
tains 64.10 per cent. of silica. 
From these and other examples it may be stated as generally 
true that volcanic sand is composed essentially of crystalline frag- 
ments, and contains less silica than the lava to which it belongs. 
With volcanic dust, however, the case is different. Microscopical 
examination shows that it is composed chiefly of volcanic glass 
particles; and as far as chemical analyses have been made, they 
indicate that volcanic dust is more silicious than the lava to which 
it belongs. 
That volcanic sand should be crystalline and basic, and the 
accompanying dust vitreous and acidic, as compared with the lava 
9 
