Naples, so far as Earthquake is concerned, whether coming from the throes of Vesuvius or elsewhere, has a pretty good chance of safety. She may possibly (though not probably) be some day smothered in ashes; but is in little danger of being shaken to the earth. During this time there have been taking place, larger eruptions of Vesuvius and earthquake shocks from other centres, together probably about the same number of times as the numbers of those years, when those columns have been more or less shaken.
We may therefore affirm that the probability (on the basis of this experience _only_) is, say 120 to 1, that the next shock, whether derived from Vesuvius, or elsewhere, that may shake Pozzuoli, will be one less in power than would be needed to overturn the shafts of the Temple of Serapis there.
Let us now turn to the second branch of our subject--viz., Vulcanology--upon which, as yet, we have secured less firm standing ground than we have seen we possess in Seismology, for which reason we took that first into consideration.
It is the part of Vulcanology to co-ordinate and explain all the phenomena of past or present times visible on our globe which are evidences of the existence and action, whether local or general, of temperatures within our globe greatly in excess of those of the surface, and which reach the fusing points of various mineral compounds as found arriving, heated or fused, at the surface.
The stratigraphic geologist sees that such heated or fused masses have come up from beneath, throughout every epoch that he can trace; but he cannot fail to discern more or less a change in the order or character of those outcomings, as he traces them from the lowest and oldest formations to those of the present day. He sees immense outpourings of granitoid or porphyrytic rocks that have welled up and overflowed the oldest strata--huge dykes filling miles of fissures that had been previously opened for the reception of the molten matter that has filled them, and often passing through those masses of previously outpoured rock; later he sees huge tables of basaltic rock poured forth over all. One grand characteristic common to all these--commonly called plutonic products--being that, whether they were poured forth over the surface or injected into cavities in other rocks, the movements of the fused material were, on the whole, hydrostatic and _not explosive_.