Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 153 
in devoting their whole attention to massive rocks instead of to the fragmentary 
ejecta of volcanoes. 
I shall attempt to recapitulate my theories in as concise a form as possible, 
and see whether they are borne out by the Vesuvian manifestation under con- 
sideration. If we examine the essential ejecta of either a paroxysmal or explosive 
eruption, we find that the first material ejected, be it scoria or pumice, was 
accompanied by the greater evolution of gases, was more vesicular, more vitreous, 
and less microlitic, and devoid of post-eruptive crystallization. The explanation 
of this rock-structure which I gave was that the fused rocks of the magma in 
the upper portion of the canal, where the walls were aquiferous, were able to 
take up and dissolve, in relation to time and pressure, the largest quantity of 
water and any salts accompanying it. At the moment of the outburst and the 
relief of pressure, the enormous loss of heat-energy in the passage of super-heated 
water 7 solution, and therefore of small bulk, to the state of voluminous expansion 
of steam, meant the rapid solidification and cooling of that portion of the magma 
which had held it. The fragments of frothy, otherwise pumiceous or scoriaceous 
magma, therefore, will be so rapidly cooled that the residual glass, in which are 
enveloped the intra-telluric crystals, will have little or no time to devitrify into extra- 
telluric crystals or microliths, as in the scoria a (figs. 26, 27, Pl. XVIII.). As more 
magma rises in the volcanic chimney, it will have come from greater depths, where 
the enclosing walls will be of less aquiferous rocks, and where also its initial 
temperature will be higher from less extensive loss either to the walls, or in the 
act of dissolving out water from them. The amount of loss of heat-energy from 
expansion of the smaller quantity of water dissolved in it will likewise mean 
slower cooling, slower vesiculation, slower ejection, and greater time for residual 
glass to devitrify or individualize into extra-telluric crystals and microliths, so 
that a darker, harder, heavier scoria or pumice will overlie the lighter-tinted, less 
heavy, or more vesicular ejecta that had fallen around the volcanic rent as 0d 
scoria (figs. 28, 29, Pl. XVIII.). 
Later, as a magma still more removed from saturation by water rises from 
greater depths, the loss of heat-energy will be still less, more time will be given for 
extra-telluric crystal and microlitic individualization (essential scoria of ¢, fig. 30). 
This crystallizing and microlitizing process may go on to such an extent that the 
entire conversion of the residual glass may be so complete as to abolish its function 
as a cohesive cement, and the whole magma-mass may issue as an incoherent 
crystalline dust. Such dusts are the final phase in many explosive and in some 
paroxysmal eruptions. It is out of place here to touch upon the accessory or 
accidental ejecta that may in greater or less quantity be carried out of the crater 
during such violent eruption.* 
* Consult my paper, ‘‘ On the Fragmentary Ejectamenta of Volcanoes,”’—Proc. Geol. Assoc., vol. ix., 
pp. 421-432, 1886, 
