172 Jounston-LAvis—The Eruption of Vesuvius in April, 1906. 
ejecta as masses ranging from the size of a walnut up to that of a man’s head or 
larger, with a hardened denser surface, showing slight bread-crust structure. The 
material is a dense black or purple-brown pumiceous scoria, rich in phenocrysts 
of augite, leucite, and other essential minerals, and often extraneous fragments, 
with which, no doubt, it was churned. 
In structure the material has a much finer vesicular structure, as seen in fig. 30; 
but its blackness from pulverulent magnetite in the matrix of the mass is most 
marked. The phenocrysts are the same as those of its two predecessors; but I 
think there are perhaps more mica crystals, though this may be due to the 
diminished bulk from the smaller vesicles, and therefore the greater crowding of all 
the porphyritic crystals. There also seems to be more microlitic devitrification 
of the glassy base. 
A comparison of the size and number of vesicles in sections (figs. 26, 28, 
and 30, Pl. XVIII., or, again, figs. 27 and 29) shows that as the magma rose 
from greater depths it was less and less rich in water. 
These words had already been written when I received the memoir of Prof. 
Lacroix, in which he gives an analysis of these scoriz, comparing the figures with 
those of the lava. No mention is made as to which band they were derived 
from, and no precautions appear to have been taken to average samples from a 
long stretch of country over which they fell. 
From this analysis Lacroix concludes that the magma that produced this 
pumice was different from that which flowed as a lava; and it is given one of 
the new rock names. Had the precautions mentioned above been taken, or had 
due credit been given to aerial sorting of fragmentary ejecta, I think that the 
‘author would not have come to such conclusions. I have over and over again 
shown how the alkalies are modified in quantity in different parts of the contents 
of an open volcanic chimney. By such change of conditions, we have more than 
sufficient explanation found in a single lava-stream. Above all, I must enter a 
protest against the injury to true science by the use of excessive nomenclature 
that hides facts, confuses our thoughts, and misleads our investigations.* It is 
irrational for different names to be given to the crust of lava, to portions of it 
that fall into a well, to the material that cooled in its main mass, or again to 
other portions of the same magma ejected as fragments, because the vapour 
contents were greater.T 
We have other analyses showing that associated with the insoluble part of 
the lava and scoria there is a considerable proportion of soluble salts; but the 
analysis given by Lacroix makes no mention of these as to whether they are 
estimated in the bulk analysis or not. 
* “ Ktude Minéralogique des Produits silicatés de l’Eruption du Vésuve (Avril, 1906),” &c,—Nouvelles 
Archives du Museum, 4 série, tome ix., p. 28. 
} Op. cit., p. 151, 
