Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 171 
crystalline form; nor do they sufficiently polarize to enable us to determine their 
nature: hence we can only judge of their constitution by analogy. A few are in 
the beautifully neat diamond-like plates, such as I described in the 1885 lava, and 
considered as felspars. In some spots the glass is absolutely devoid of iron ore 
microliths ; but in others, more or less abundantly, may be seen dark-brown 
crystallites which, also by analogy, we refer to that mineral. 
We glean from examining this scoria, that the magma from which it was 
derived was composed in very considerable part of a glass fairly charged with 
H,O* in solution, acquired in the higher, though not the top, part of the volcanic 
chimney. As soon as the great plug of lava above it had rapidly been removed, 
as a cork from a lemonade-bottle, it frothed and scummed up by the rapid escape 
in a gasified form of its dissolved H,O, accompanied, of course, by an enormous 
explosive power, and, above all, by a rapid loss of heat-energy requisite for that 
gasification or vesiculation. No time was allowed for much further individualiza- 
tion before the glass set. 
_ Deeper down in the volcanic chimney was other magma, which had less 
opportunities for acquiring H,O. This rose, following that portion of the magma 
that supplied scoria (a), and was ejected, constituting the black scoria-lapilli (0). 
-Macroscopically, it much resembles its eruptive predecessor, but is denser, always 
sinking in water; it is blacker, and the average size of the vesicles is smaller. 
Compare figs. 26 with 28, and 27 with 29. Even in the very thinnest section 
that human ingenuity can prepare, it appears microscopically as a jet-black 
network (fig. 26), with here and there near the edges of the slice a fortuitous, 
almost infinitesimally thin, wedge-like edge, just penetrable by a very powerful 
light from the sub-stage. Such a spot is photographed in fig. 29. It differs from 
the first ejected scoria (a) by the glass being choked by vast numbers of augite, 
possibly felspar, microliths, and iron-ore dust. ‘To this latter is especially due the 
intense black opacity of this scoria. 
Being a less gas-bearing magma, its cooling was slower, and consequently 
time was given for the more extensive though hasty individualization of the 
potentially existing minerals. s 
Essential ejecta—which, no doubt, did to a small extent quit the throat of the 
volcano during the production of the red and brown lapilli-beds and the chocolate 
dust—are very difficult to recognize, on account of their acquiring an old look 
from attrition with the other lapilli. They are then indistinguishable from scorias 
of earlier eruptions that in part constitute these accessory gecta. By carefully 
selecting pieces from some of the Atrio sections, close to the foot of the cone, but 
just beyond the reach of the slips of loose ejecta, I was able to recognize the 
* T always use the formula, not to denote any special physical state of the substance. 
TRANS. ROY, DUB. SOC., VOL. IX., PART VIII, 2H 
