Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/243

 which in both amounts nearly to the angle of the dip of the outward surface of the hill which they compose.”

“ These differences seem to have resulted from the greater vicinity of the cliffs at Somma to the seat of the crater as well as to the more rapid slope of the hill, which from the greater degree of fluidity of the lavas, and their more rapid tendency to descend, did not allow them to acquire there any considerable thickness; whereas in Antrim the distance being probably greater from the crater, and the slope much less, the lavas could settle there in greater depth. The basaltic walls also found along the coast of Antrim, and particularly of Ballycastle and Belfast, seem to have a perfect analogy with those of Somma, but are of much greater breadth in general, and the intervening strata of porous and irregular basaltic matter between them correspond exactly with those of scoriæ in all lavas, and so visible in the ancient ones of Somma.”

“ In the side of one of these walls of Somma, I found a crust of completely vitrified matter, covering a schistose cracked and very fragile homogeneous lava, disposed, contrary to the general rule of that kind, in perpendicular joints, and much resembling a kind of schistose hornstone, as well as the upper and superficial covering of Pleaskin.”

“ The whole of the valley between Somma and Vesuvius is covered with repeated irruptions of lava, particularly those of 1767, 1779, and 1787, which have run to the foot of the rock and to a considerable depth.”

This description will be found to agree pretty well with that of M. Breislack (Voyages dans la Campanie. tom. 1. p. 133. Paris 1801.); from both of which it is sufficiently clear, that the walls of Monte Somma are of the same nature with the dykes of the north of Ireland.