OF MONTE SOMMA AND VESUVIUS. 81 
in earlier examples, namely, that the eruption commenced by the vio- 
lent ejection of vitreous matter containing only a few crystals of sani- 
dine, amphibole, and garnet. ‘The second, or perhaps all of these mine- 
rals, would lead us to believe in their formation at a time precedent to 
their ejection, deep below the surface and under great pressure. So 
rapid was the expulsion and cooling of these earlier materials that 
they have retained their vitreous and vesicular structure. As this 
primary matter was hurled into the air, it carried with it the rem- 
nants of lava and scoria that during the preceding tranquillity had 
erumbled in and partly filled the crater. 
After the first paroxysm there seems to have been a lull, during 
which the finer ash-bed was deposited, and during which a more 
tranquil ebullition was going on, allowing the gradual escape of 
heat and vapour, so as to favour crystallization. 
The bursting through of the superincumbent pressure and conse- 
quent explosion, no doubt, must have cleared the vent to enormous 
depths, and the lull or temporary diminution of violent eruption 
would be coincident with the more gradual welling-up of new mate- 
rial from below. This, as it rose to the surface, would gradually 
-be reduced in temperature from the regular expansion of the aqueous 
matter, so that time would be given for its vitreous base to develop 
into “formed” materials, or in other words, the microliths of biotite 
and pyroxene. The essential or primary eruptive material of the 
middle subdivision, characterized by its more crystalline nature and 
less vesicular structure, shows that the elastic fluid had already, to a 
great extent, quitted it before its ejection. It is just this escape of 
aqueous and other vapour that tore off those interesting metamorphic 
rocks above mentioned, and so greatly increased the size and depth 
of the crater. 
The Apennine limestone, as we have seen, forms the platform on 
which were deposited the quaternary sandstones, marls, and clays 
which support probably very early volcanic deposits and certainly 
the majestic cone of this prehistoric voleano. It was through 
these rocks that the igneous magma found an outlet, and through 
them poured, for centuries upon centuries, that lava and its 
products which, cooling and collecting around the vent, gradually 
built up a mountain more than 2000 metres in height. During all 
this time the walls of the chimney had been bathed by a stream of 
liquid silicates of very high temperature and under considerable 
pressure, varying with the distance from the surface, &e. Such 
conditions could hardly be expected to do otherwise than bring about 
most important chemical changes in the surrounding limestone, so 
as, In some cases, to replace it by rocks totally different. It is not 
uncommon to meet with blocks of limestone which have been taken 
up by a lava-stream as it flowed over the surface, the result of which 
is that what was a carbonate becomes covered with a shell variable 
in thickness, but filled with minute crystals of pyroxene and biotite. 
Tf such effects can be produced in, at the most, a few weeks, at ordi- 
nary atmospheric pressure and lowered temperature, what should we 
Ooie Casa Nose). G 
