MECHANISM OF VOLCANIC ACTION—JOHNSTON-LAVIS. 3811 
caught up in the magma. Microscopically it is made up of a net- 
work of straw-colored glass, with innumerable minute micro-crystals 
of leucite, all of a remarkably uniform size, besides which are a very 
few scattered microliths of hornblende, augite, mica, and felspars, 
obviously preeruptive in birth. (PI.3, figs.5 and 6.) At wide inter- 
vals, of course, occur the porphyritic crystals above named. The 
main mass, however, is made up of glass, so that all the vesicles have 
been able to assume well-rounded outlines. The size of the vesicular 
spaces is very great in proportion to the amount of solid material 
inclosing them, making the pumice a very light one in weight. 
Reposing on the bottom stratum, and rather suddenly graduating 
up from it, is the main bulk of the ejecta. The pumice composing 
this is much heavier, darker in color, ranging from a brownish gray 
to a greenish gray. The porphyritic inclosed crystals are the same 
as those in the bottom, white pumice, and practically average the 
same size. They appear to be more frequent, but this is due to the 
pumice being denser; more of them are therefore to be seen in the 
same area, as it were more crowded together. 
Microscopically this pumice very much differs from the subjacent 
white variety. (PI. 3, figs. 7 and 8.) Nearly the whole of the glass 
has been replaced by innumerable microliths. The small leucites 
have increased but little in size, though they seem more numerous. 
This I attribute to the less amount of open space left by the vesicles. 
The augite microliths constitute the main bulk, and minute grains 
of magnetite are abundant. Some small microliths are probably 
distinguishable as felspars. Generally the vesicular cavities are 
much smaller. The vesicle walls are no longer smooth, but rough 
from the projecting microliths that in the process of rapid cooling 
grew and projected in all directions, and are not arranged with such 
parallelism that is seen in flow structure with rods already in ex- 
istence at an earlier date. 
Both these divisions of essential ejecta are more or less mixed with 
accessory and accidental ejecta torn from the sides of the crater and 
the subvoleanic platform. I mention this as it has a bearing on the 
composition of the third and uppermost part of the materials shot 
out during an explosive eruption. This third or uppermost portion 
consists of a coarse or fine dust (ash), and when examined micro- 
scopically is seen to be to a large extent composed of detached 
microliths and loose crystals mixed with a large quantity of pulver- 
ized accessory and accidental ejecta. Whenever one examines the 
ejecta of explosive eruptions the same order is found. The above- 
mentioned characters are better seen in the case of very fluid inter- 
mediate or basic magmas, and I have given figures of the ejecta 
of an earlier explosive eruption of Somma Vesuvius, Phase ITI, 
which is free from leucite. (PI, 2, figs. 1-4.) 
45745°—sm 1909——21 
