6o 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
Less fi'equeiitly evidence is obtainable that the blocks were partially or 
wholly molten at the time of expulsion. Sometimes, for example, a mass 
which presents on one side such a broken lace as to indicate that it came 
from already solidified material, will show on the other that its steam- 
vesicles have been pulled out in such a way as to conform to the rounded 
surface of the block. This elongation could only take place in lava that 
was not yet wholly consolidated. It seems to indicate that such Idocks 
were derived from a thin hardened crust lying upon still molten material, 
and that they carried up parts of that material with them. As each stone 
went whirling up the funnel into the open air, its melted part would be 
ilrawn round the gyrating inass, and would rapidly cool there. 
In other cases, we encounter true vf>lcanic bombs, that is, rounded or 
Ijomb-shaped blocks of lava, with their vesicles elongated all round them 
and conforming to theii’ spherical shajie. Sometimes such blocks are 
singularly vesicular in the centre, with a more close-grained crust on the 
outside. Their rapid centrifugal motion during llight would allow of the 
greater expansion of the dissolved steam in the central part of each mass, 
while the outer parts would be quickly chilled, and would assume a more 
compact texture. Bombs of this kind are met with among ancient volcanic 
products, and, like those of modern volcanoes, have obviously been produced 
by the ejection of spurts or gobbets of lava from the surface of a mass 
in a state of violent ebullition. Occasionally they are hollow inside, the 
rotation in these cases having probably been exceptionally rapid. 
Passing from the larger blocks to the smaller fragments, we notice the 
great abundance of nut-lik'e subangular or rounded pieces of lava in the 
agglomerates. These include lumps of line grain not specially vesicular, 
and probably derived from the disruption of solidified rock. But in many 
agglomerates, esp(icially those associated with the o^itpouring of basalts or 
other basic lavas (as those of Carboniferous and Tertiary age described in 
later chapters), they comprise also vast numbers of very finely cellular 
material or pumice. These pumiceous lapilli have been already alluded to 
as ingredients of the stratified t\iffs. But they are still more characteristic 
of the necks, and reach there a larger size, ranging from the finest grains up 
to lumps as large as a hen’s egg, or even larger. 
The peculiar distinctions of this ejected pumice are the e.xtreme minute- 
ness of its vesicles, their remarkable abundance, their prevalent spherical 
forms, and the thinness of the walls which separate them. In these 
respects they present a marked contrast to the large irregularly-shaped 
steam -cavities of the outllowiug lavas, or even of the scoria3 in the 
agglomerates. 
'This characteristic minutely vesicular pumice is basic in composition. 
tVhore not too much decayed, it may be recognized as a basic glass. Thus 
among the remarkable agglomerates which fill up the Pliocene or Pleistocene 
vents of the A'elay, the fragments consist of a dark very basic glass, which 
encloses such a nmltitude of minute steam-cavities that, when seen under the 
microscope, they are found to he separated from each other hy walls so thin 
