5384: Dr. H. J. Johnston-Lavis—The Ponsa Islands. 
at hand, for at Ventotene it attains a thickness of 10 m. or more and 
is distinctly if at all represented at Santo Stefano. It is probably by 
this eruption that the greatest destruction was wrought in the old 
cone, and that the crater apex extended down into the subjacent 
metamorphosed rocks. In fact, the explosive products at Santo Stefano 
are generally smaller and finer than at Ventotene, and seem therefore 
to have been derived from a centre or centres nearer the latter 
island. A grave error has crept into Prof. Judd’s “ Volcanoes,” 
where the lavas are represented as a stratum covering the tuffs of 
the island, instead of the reverse. 
These different pumice deposits are chiefly derived from a basic 
magma, probably having the same composition as the doleritic 
lavas of the island. Of far greater interest from a physical point 
of view is the regular sequence of the different ejectamenta in each 
explosive eruption, which I first pointed out at Vesuvius, and have 
confirmed in a very large number of volcanic deposits. In each we 
find comparatively vitreous pumice at the bottom followed by micro- 
crystalline pumice or scoriaceous pumice, and finally a dust bed. 
Whilst the divisions show a rapid increase of extratelluric ‘‘ formed 
material ” as the eruption progressed, the intratelluric minerals have 
not increased in number, the whole confirming the conclusion that 
the hydration of the igneous magma was limited to near the earth’s 
surface.” 
The blown sand already mentioned consisted of a mixture of 
augite and other mineral grains with shell fragments and foraminifera. 
It must have been carried up on the slopes of the island by the 
wind from an old beach probably formed on the extensions of the 
last great tuff deposit, but now cut away. By percolation of 
carbonated waters, nearly all the subjacent rocks have been 
permeated and often cemented together or filled by calcareous 
concretions. We find the lavas near the sea-level also become 
amygdaloidal by the presence of tufts of aragonite crystals in the 
vesicles. 
Very interesting is the presence of irregular compact white 
amorphous concretions, often attaining a weight of 50 or more 
- grammes, and composed of almost pure magnesite. These are 
included in and between the masses of scoria of the buried cone of 
eruption. They are probably due to emanations at and subsequent 
to the eruption. I have noticed very similar ones in a vesicular 
doleritic lava of the east coast of Vulcano. 
The next point to which I wish to draw the attention of geologists 
is the remarkable evidence of elevation to be seen in the island of 
Palmarola, which seems to have escaped the notice of recent visitors. 
In the middle of the island is a depression somewhat in the form 
of an amphitheatre, open to the west and shelving down to the small 
' The quantity of these explosive ejectamenta is such that the scoria cone was 
entirely buried in them, so that its existence is only known by the exposures in the 
cliff sections. See Figs. 1 and 2. 
* Q.J.G.8. 1884 and Proc. R. Dublin Soc. vol. v. July, 1886; Proc. Geol. 
Assoc. vol, ix. 
