Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 193 
and some efforts at repair as the rock cooled. Other rocks such as these also 
contain nepheline, augite, amphibole in very varying proportions, that could 
undergo fusion when exposed to suitable conditions, especially with alkaline 
chlorides or sulphates as fluxes. It is obvious. therefore, that such a black glass 
would have no very definite composition ; but in either case we should be prepared 
to find an increase in the alkalies such as the analyses show. The other marked 
feature in the composition of this glass is the diminution of lime and magnesia, 
for which several hypotheses might be offered. 
Lacroix* does not admit this black glass to be a result of partial re-fusion of 
the rock, since, he thinks, it would not recrystallize with much larger and finer 
crystalline contents, and also that it would be a more basic material in composition. 
The facts and the remarks I have above made about them will, I think, quite remove 
those difficulties. Besides, I have often collected blocks of semi-fused old lavas 
having the same characters as the above-mentioned ones, and that were ejected 
and fell at my feet, with Vesuvius in its chronic lava-cake, cone-forming stage. 
Specimens of these in my museum are a nuisance on account of the constant 
efflorescence of chlorides from their surface, and such efflorescence occurs from the 
cracks of this black glass under discussion. Even in the tachylytic glasses there 
is a marked difference in composition, especially in the iron. Magnesia and soda 
also vary considerably. 
Rather frequent in ejecta of this eruption were large or small masses of a 
spongy, vesicular glass—a cooled, glassy froth, in fact, either filling clefts and 
cavities in old blocks of lava, but more frequently enveloped in a crust of new 
compact lava, so constituting a volcanic dumpling. Sig. Matteucci also records 
their presence. I have collected similar material in the contents of dumplings 
of 1872 and other eruptions. Sig. Matteucci gives an analysis, showing this 
substance to be largely a lime and alumina silicate. + 
Brock or Levcrrire with Virreous SEPARATION AND OF Lime SILICATE. 
*¢ Pumiceous ”’ 
Crystalline Part. Vitreous Part. 
$10, : 0 : 48°372 58°508 
FeO . . : 4°904 5°300 
Fe,05 c : . 4:500 3°556 
Al,03 (with P,O;) 16°372 10°313 
CaO 6 : . 11°611 12°648 
MgO : : ° 6-916 1°506 
K,0 : 0 Q 5°147 3°398 
Na,O : . c 1°448 3°900 
These masses I met with of all sizes, ranging from that of a nut up to a bomb- 
like block 0:60 m. in diameter. They were usually coated with a black crust of 
* Op. cit., p. 42. t Op. cit, 
