90 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
troversy that arose (and was supported by Pilla, Tondi, Tenore) as 
to the aqueous deposition of the pumice, &c¢., which envelops Pompeu ; 
the whole of the arguments in favour of this view being cleverly 
refuted by Prof. A. Scacchi. An abstract account may be found in 
Lyell’s ‘ Principles’ and in some original papers of Scacchi *. 
So far we have said little of Herculaneum. The lower part of 
the tufa that covers this city seems to have fallen from the air in 
the same manner as at Pompeii, and only to have been enveloped in 
the mud at a later period of the eruption. How it was that such 
an enormous deposit of ‘lava d’ acqua,” as itis locally called, collected 
at this particular spot, is a somewhat difficult question to answer ; 
there may have been above the town the lower extremity of some 
considerable vallone which may have collected the mud, and directed 
it upon the doomed city. In the chapter on denudation, it is 
shown what a sharp shower of rain can do in the way of transporting 
materials; but when we consider the enormous amount of vapour 
arising from one of these paroxysmal eruptions mixed with falling 
and loose pumice, we are aided in the comprehension of this some- 
what extraordinary phenomenon. 
As this eruption is really the first one distinctly recorded in history, 
and as its pumice is characterized by the presence of leucite in 
great abundance, I have thought it convenient to group it with 
others its direct successors, under the head of Puasz VIL., all of 
them being rich in microliths and crystals of leucite as components 
of the primary or essential material. This leucitic condition of the 
pumice is a character of no little importance in the study of the 
formation of volcanic rocks. Messrs. Fouqué and Lévy t, in making 
a leucotephrite, found that the leucite crystallized at a white heat, 
but they had to reduce the crucible to a red heat before the felspar 
separated out. 
We must suppose that the magma that gave rise to this eruption 
had by the gradual escape of heat undergone the crystallization of | 
its leucite, since this mineral does not seem to have increased in 
size or number from the very beginning to the end of the eruption. 
We should therefore expect the temperature to have been lower 
than on former occasions, when the sudden ejection and cooling 
prevented the development of this mineral. ‘ 
The experiment of the artificial formation of rocks is thoroughly 
borne out, when it is stated that the felspar would only crystallize 
at a red heat, since of all the minerals it seems to be one of the 
last formed in the pumice, being later than amphibole, magnetite, 
biotite, and pyroxene in some cases. There is a fact, however, that 
is hardly reconcilable with the above-mentioned relation of the two 
minerals under discussion. In a later eruption a rock was ejected 
in which the leucites may be as large as walnuts, and often partly 
or entirely envelop large crystals of sanidine and pyroxene. 
It seems to me that prolonged “ recuite” at ordinary atmospheric 
* “ Osservazioni critiche sulla maniera come fu seppelita l’ antica Pompei,” 
and ‘‘ Bollettino archeologico Napolitano,” no. vi. Marzo, 1843. 
tT ‘Synthése des minéraux et des roches,’ p. 65. 
