164 Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 
in thin section both are very pale bottle-green in colour. The leucites are large, 
composite, clean crystals, with very few enclosures, at any rate in the larger ones. 
The augites and leucites are, as very frequently happens, intergrown, though the 
augite seems to have started individualizing earlier than the leucite. The 
pectinoid polarization of the leucite is only moderately marked in the interior of 
the stream, but exhibits more contrast of light and shade near the rapidly cooled 
surface, or that suddenly cooled by falling into a cistern of water at Boscotrecase. 
A few olivines or biotites are occasionally met with; in fact, I feel inclined to 
consider the amount of this latter mineral somewhat above the average for 
Vesuvian lavas of recent dates. 
The matrix consists of a closely packed mass of small leucite crystals fairly 
well defined, showing usually one or more rings of enclosures, but otherwise 
limpid. These are enveloped in a thick network of felspar and augite microliths, 
with small magnetite grains, but no magnetite dust, so that sections are fairly 
transparent. In the sections taken from the interior of the stream, most of the 
residual glass seems to have been completely individualized. 
What strikes one is the fair uniformity in the limit of size of the post-eruptive 
leucites. There are a certain number of crystals and crystal groups of all inter- 
mediate sizes, but the frequency of their occurrence is relatively small. (See figs. 
22, 23, 24, 25, Pl. XVII.). 
Another point is the relatively small size and unimportance of the felspars. 
A general glance at the ground-mass impresses one with the fact that the vast 
number of small leucites constitutes the dominant constituent of it. 
I thought it wise to compare these new lavas with those produced under 
different conditions, in the slow and steady outpours that built up the great lava 
cone of the Colle Umberto. Materials were collected from one of the latest out- 
flows on the western slope of the Colle Umberto, and therefore emitted about 1899. 
A section of the interior of this stream (figs. 20 and 21, Pl. XVII.) shows the 
larger leucites to be very similar to those of the lava of April, 1906, but they 
grade down to quite the smallest leucite, so that there is no marked character 
either in size or otherwise to differentiate between the intra- and extra-terrestrial 
individualization of this mineral. A further more striking fact is, that the vast 
number of small felspars constitutes the dominant constituent of the ground-mass of 
1895-1899. The magnetite graims in the 1899 lava are also larger, and, as far as 
one can see under high powers, are more perfectly shaped. 
These somewhat striking differences are quite capable of interpretation. For 
four years lava remained in the chimney of Vesuvius, from 1895 to 1899, ina 
comparatively quiescent condition, steadily rising along the main canal of the 
volcano, and then draining away through a narrow dyke or fissure, until it 
found its outlet at the summit of the Colle Umberto. For a kilometre and a hali 
