OF MONTE SOMMA AND VESUVIUS. oT 
vapour to find a passage through it, which in escaping would carry 
the smaller fragments with it, and, depositing them on the outer 
slopes of the cone, would very soon clear the vent. The larger 
masses, if too great, would be subjected to such powerful grinding 
movements that they would soon be reduced to finer fragments, and 
would then gradually be ejected. It also presupposes only one 
paroxysmal eruption as the cause of all this great cavity, whereas 
a number did the work piecemeal. Phillips* accounts for it by 
considering the southern side of the mountain weaker, as proved by 
all recent lateral eruptions, which burst forth from that portion. 
It seems that there is no reason why one side of the mountain 
should be weaker than another. This tendency of dykes to reach 
the surface first on the south side is undoubtedly due to the modern 
eruptive axis being in much closer vicinity to it. 
Von Buch’s 7 opinion of the matter is much the same, except that 
he is trammelled by his pet hypothesis. 
The mention of the southern side of Monte Somma as being either 
the lowest or weakest point is assuming a condition as necessary 
which it is intended to explain. 
Even in that unique and peculiar voleano the Puy de Chopine ¢, 
near Clermont, if the eruption had been as violent in character as 
some of those that excavated the crater within Somma, the obstruct- 
ing mass of granite would have been carried away. 
A change of eruptive axis is a thing not at all uncommon, and 
dozens of examples may be given; but two remarkable ones of the 
Italian group Etna and Volcano are sufficient§. From day to day 
we may see the same process going on, on a smaller scale, in the 
crater of Vesuvius. Even now the vent opens right away in the 
K.N.E. corner of the 1872 crater. 
Some authors|| have supposed that the principal part of the Vesuvian 
cone was thrown up by the eruption which destroyed Pompeii. Be- 
fore accepting such a theory, it is worth while considering some 
general facts bearing thereon. 
Let us imagine the condition of affairs towards the termination of 
the Plinian eruption, which had pared the sides, and cleaned out 
the bottom of the great crater of the Atrio del Cavallo. The centre 
of Somma was then occupied by this vast conical hollow which had 
been cleared out by explosions that were capable of carrying lapilli 
from 50 to 100 grammes in weight a distance of 22 kilometres, 
and depositing them on Mt. St. Angelo, nearly 1000 metres higher 
than the lower edge of the crater. | 
* Vesuvius, 1869. 
T ‘ Description physique des iles Canaries.’ ‘Traduit par C. Boulanger, 1836. 
t Serope, ‘ Volcanos,’ 1822, p. 71. 
§ Lippi pointed out many years since the remarkable fact that all the prin- 
cipal Italian voleanos, as Roccamonfina, Ischia, Vesuvius, and others, showed 
great craters truncated lowest to the seaward (‘ Geologia volcanica della Cam- 
pania del Dottor Nicola Pilla.’ Naples, 1823). ‘The same has been observed in 
other districts: see Charles Heaphy ‘‘ Volcanic country of Auckland, New Zea- 
land,” Quart. Journ. Geol. Soe. vol. xvi. p. 244. 
|| Palmieri, ‘ Il Vesuvio e la sua Storia,’ 1880, p. 9. 
