82 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
expect to be the condition of the limestone which forms the walls of 
a voleanic chimney ? 
So high, indeed, has been the temperature that sometimes a rem- 
nant of limestone in a mass of pyroxenic or other rock had been 
completely fused, so as to form so many beads covering the walls of 
the cavity. We know from the experiments of Sir J. Hall on 
calcic carbonate, that this substance only fuses at very high temper- 
ature, and under such pressure as prevents the escape of the carbonic 
anhydride. 
Another example is that of geods of leucite crystals in an altered 
limestone, rather common among these ejectamenta, which must 
have, undoubtedly, been deposited by fusion, in addition to which the 
crystals usually show signs of part refusion. Knowing the very 
refractory nature of this mineral, we may form some idea at least of 
the temperature that some of the limestones only slightly altered 
have undergone. 
The earlier eruptions, and especially the first of this PHAsE, we 
have shown to have already formed a crater of large dimensions 
whose apex had reached into the subjacent Tertiary beds, the 
remnants of which we showed to be commonly mixed with the- 
primary materials. This eruption we have been studying was one 
of the most violent, if not the most violent, of paroxysms, that had 
been tearing out the heart of the mountain, and added not a little 
to extend what had already been accomplished. By re-paring the 
walls of the crater, it had still more enlarged it, and carried the 
apex deeper into the sedimentary limestone that formed the base 
on which reposes the volcano. 
Any one who makes himself acquainted with the resulting deposits 
of this eruption cannot but be astonished at the size and abundance of 
the blocks of limestone and its derivatives. I measured on one occa- 
sion in a section a block of limestone which had a face of 2 x 3 metres ; 
and there are many blocks of lava about a metre in diameter, showing 
with what power they must have been torn off and hurled forth. If 
we attempt to form a conception of the amount of sedimentary rock 
that has thus been broken up, we arrive at the conclusion that the 
crater produced by this paroxysmal eruption must have extended 
deep into the underlying limestone. It has already been shown that 
limestone must exist at a considerable depth below the Campanian 
plain, which is but a few feet above sea-level, and from which this 
volcano rises. We see, therefore, that this craterial hollow must 
have extended far below the level of the sea, thus confirming the 
calculation already derived from the ratio existing between the 
breadth and depth of a crater. 
At the latter part of the eruption, when the expansion of aqueous 
matter went on more gradually, and the development of the vitreous 
magma into a crystalline form was complete, the absence of the 
amorphous base would render the cohesion between the crystals and 
microliths much feebler, so that, as expansion continued, the whole 
might be reduced to ash. Even were the cohesion not entirely 
destroyed, yet we may suppose it to be much reduced, so as to render 
