170 Jounsron-Lavis—The Eruption of Vesuvius in April, 1906. 
As we approach the cone, the thickness of fragmentary ejecta rapidly 
increases; but the different disturbing influences were so great that little can be 
learned by excavating sections of the deposits. Vast quantities of ejecta that col- 
lected on the outer slopes of the cone continually slipped down as great avalanches 
sweeping out over the Atrio, mixing their debris with that on the spot where the 
moving masses come to rest, and so destroying any order of stratification. We shall 
more fully enter into this subject when describing the changes of the great cone. 
If we select a number of pieces of the lower and first essential ejecta, we find 
them composed of a fairly light, spongy scoria, with a faint tendency to pumti- 
ceous structure. Their volume varies according to the distance from the 
eruptive axis, but at Ottajano pieces up to the size of a pigeon’s egg were 
common. Ags I pointed out years ago, the size and density increase as we 
approach the eruptive centre in the azimuth of the shower. At Ottajano, a small 
proportion of the large pieces will float in water. Were it necessary to make a 
bulk analysis, it would be requisite to collect specimens from the whole line of 
fall, and take an average from these. 
Projecting into the cavities of the scoria, we can occasionally see a nearly free 
crystal of augite, of leucite, and rarer of olivine and mica. 
The making of microscopic sections is a difficult task, as in all vesicular 
rocks; further, the opacity is so great in this case that the grinding must be carried 
to extreme thinness to render them sufficiently transparent. Fig. 26, Pl. XVIII., 
gives a representation of the appearance when magnified 174 diameters. The 
vesicular structure is characteristically that of a scoria and not a pumice. A 
very large proportion of this scoria is composed of a buff or faintly greenish 
glass, scattered throughout which are large numbers of porphyrite leucites, 
augites, and the other usual mineral components of Vesuvian essential ejecta, 
fragmentary or otherwise. There is nothing very special about any of these, 
except that most of the larger augites are much more crowded with glass 
cavities than the lavas. They are darker bottle-green in tint, and -zoned, 
and also polarize in brilliant and variegated colours from irregular and rapid 
cooling, independent of these zones. There are also some light augites like 
those in the lava. The larger leucites are remarkably clear, and polarize 
rather strongly, such as would be expected with the rapid cooling of the 
scoria. The medium and smaller-sized leucites, down to the very minute 
ones, exhibit neat, well-formed trapezohedrons, that are well seen in the patches of 
clearer glass by the constant use of the focussing screw. With higher powers 
(fig. 27) beautifully formed microliths of great minuteness can be seen as 
completely crystallized as those we can collect half a centimetre in length. Of 
course, they are too small to show any appreciable polarization. Numbers of other 
rod-like microliths are probably the felspars; hut here there is no recognizable 
