74 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
aqueous matter in the form of spray or vapour, derived from the 
inpouring through the walls of the crater. May it not be that 
towards the end of the eruption of the white pumice the continual 
enlargement of the crater had advanced sufficiently far to produce 
the conditions favourable to the inflow of water? In such a manner 
we could easily account for the superficial re-sorting of materials, the 
pisolitic concretions, and the breccias in the bottom of old valleys. 
But now arises the difficulty of the vacuoles in the concretionary 
bands. They may have been formed by the coating of drops of water 
falling into dusty matter, but more probably by chemical action. 
The rain derived from the condensation of the vapours of an eruption 
is rich like them in hydrochloric acid, and is well known to be the 
great destroyer of plant life for this simple reason. If we figure to 
ourselves such a weak acid solution falling amongst volcanic ashes 
containing minute fragments of calcareous rock intermixed, the 
consequence is evident. The bubbles of carbonic anhydride liberated 
would remain enveloped in the pasty mud, leaving their traces as the 
cavities we now see. At the same time the pisolitic concretions sepa- 
rated out, and these, from their abundance, crowded upon each other, 
and so converted the whole into the compact masses we have now 
been considering. I have collected materials quite similar on the 
slopes of Etna; and the same may be seen in other volcanos of the 
Campi Phlegrei. May it not be that these segregations are analo- 
gous to those which occur in certain felspathic glazing-creams used in 
pottery, which form concretions unless kept constantly in motion ? 
Whatever may be the cause of this peculiar structure, the example 
is one of considerable importance in the study of ancient volcanic 
rocks. Let us imagine these extensive sheets of concretionary rock, 
having the same inclination as the slopes of the cone, interbedded 
between other volcanic ejectamenta, still more altered than it is at 
present, its vesicles filled by zeolites, after having been buried under 
the pressure of superincumbent rocks. Afterwards suppose these 
strata cut down to and exposed in section by subsequent denuda- 
tion, so as to be within reach of a geologist or petrologist. Hither of 
these sczentiates might be sorely puzzled as to whether the rock was 
a decomposed lava or an altered ash. The petrologist might point 
to its minute structure, show that the microliths were enveloped in 
a “ devitrified”(?) matrix, and then confirm his assertion by the 
amygdaloid condition. The geologist, if it were possible, might 
point out the concretionary appearance of the bed, its remarkable 
thinness and uniformity over large areas, and no collection of the 
material in hollows, such as flowing lava would produce. Altogether 
I rather fancy that the former individual would get the better of 
the argument, and geological science would have blundered. 
These considerations suggest to our minds many such geological 
enigmas of quite recent date. As good an example as any perhaps 
is that of the rocks of the Brent Tor volcano discussed by Mr. F. 
Rutley *. 
* “On the Schistose Voleanie Rocks occurring on the West of Dartmoor, 
with some Notes on the Structure of the Brent Tor Volcano,” Quart. Journ. 
Geol. Soc. vol. xxxvi. p. 285. 
