192 Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 
Esectep Brocxs oF Otp Lava or Dyxs-rocx contarntnc PatcHEes or Brack Guass. 
Leucitite Matrix, | TLeucittephrite | Compact Vitreous) Vitreous Portion. 
Matteucci. Taare. Matteucci. Lacroix. 
BO; 2 o o || cyrimi 48-20 51-808 53°10 
THO. 5 6c 4-554 4°30 7-197 4°77 
MO} eo « 5323 3:29 i: 0:07 
MO; o o 17-340* 18°12 20°159 20:70 
GO oo ¢ 9-438 8-40 2-941 3°18 
MeO... 4-976 4:64 0596 1:77 
Keo, diveh 7-739 8-99 5°709 5-84 
MEMO 5 2) 2°355 251 11-212 | ~~ 9-10 
AOR be le | |. oo 
UO Go) 6 ko 1:59 as | 0-47 
Piosaid Wad ae 0:38 = | i 
| 99-176 100-42 99622 | 99-70 
| | 
From these it will be seen that the amount of silica and soda is greater than 
in the matrix rock and in recent lavas. The question arises therefore as to the 
origin of this tachylitic glass that fills these cavities. I quite agree with 
M. Lacroix, that it is not due (in most cases, at least) to partial refusion of the 
matrix.f It may be a residual glassy juice, derived from the fluid magma that 
must have existed in the precincts of this cavernous matrix when that was 
penetrated by this glass. It is more likely, I think, to be due to two other possible 
causes. The first is the fusion of silicate and other minerals that sublimed and 
were deposited in these fissures and cavities, and later by a rise of temperature 
and the presence of alkaline chlorides or sulphates as fluxes were re-fused.{ I have, 
in fact, frequently met with such partial fusion of minerals in geodes in ejected 
blocks of this region. In other cases such tachylytic glass may be due to the 
fluxion of sublimed alkaline chlorides with some of the leucitic material. This 
matrix-rock is quite similar to those that are frequently crowded with zeolitic 
deposits, the re-fusion of which with a certain amount of the minerals forming the 
wall of the cavity may easily furnish a glass of this composition. In fact, some of 
the figures given by M. Lacroix§ show partial fusion of the rock-forming minerals 
* Also with P.O;. { Op. cit., p. 45. 
{Since writing the above some of these specimens are thickly crusted with alkaline chlorides which 
have effloresced from the pores of the rock. § Op. cit., pl. 11. 
