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Beneath the POUZZOLANA a thin layer of humus contains fragments of pottery of Hellenic origin; which marks the close of the historic period, and covers over the mass of pumiceous tufa vomited out by the volcano. It was in this tufa, which is eight feet thick, that the first signs of buildings were discovered. Further excavation brought to light two houses with doors, windows, and bearing walls.
The white, pumiceous, trachytic tuff-like varieties are of rather greater specific gravity than the pumiceous mudstone on the coast to the north; some of the layers, especially the browner ones, are coarser, so that the broken crystals are distinguishable with a weak lens. The layers vary in character in short distances.
We were now within the embrace of a broad bay flanked on either hand by a low promontory. The beach was of dull-grey sand, and sloped steeply up to a ridge, perhaps sixty or seventy feet above the sea-level, and irregularly set with trees and undergrowth. Half way up was a square enclosure of some greyish stone, which I found subsequently was built partly of coral and partly of pumiceous lava.
Under the blowpipe, it behaves like the specimens from Ascension; but it is less hard and glossy, and the surface has not the shrunk appearance. Basaltic lavas. Numerous craters truncated on the same side. Singular structure of volcanic bombs. Aeriform explosions. Ejected granitic fragments. Trachytic rocks. Singular veins. Jasper, its manner of formation. Concretions in pumiceous tuff.
These beds are covered by others of a peculiar soft white stone, including much gypsum, and resembling chalk, but really of a pumiceous nature. It is highly remarkable, from being composed, to at least one-tenth of its bulk, of Infusoria. Professor Ehrenberg has already ascertained in it thirty oceanic forms.
AA. Superficial bed of reddish earth, with the remains of the Macrauchenia, and with recent sea-shells on the surface. B. Gravel of porphyritic rocks. C. and D. Pumiceous mudstone. Ancient tertiary formation. E. and F. Sandstone and argillaceous beds. This case, though not coming strictly under the Pampean formation, may be conveniently given here.
The white pumiceous mudstone, with its abundant gypsum, belongs to the same general epoch with the underlying fossiliferous mass, as may be inferred from the shells included in the intercalated layers at Nuevo Gulf, S. Julian, and S. Cruz.
Secondly, small irregular nodules of the obsidian, either standing separately, or united into thin layers, seldom more than the tenth of an inch in thickness, alternate repeatedly with very thin layers of a feldspathic rock, which is striped with the finest parallel zones of colour, like an agate, and which sometimes passes into the nature of pitchstone; the interstices between the nodules of obsidian are generally filled by soft white matter, resembling pumiceous ashes.
Walking through the first of these you clambered down some slippery stairs into what was once a breathless subterranean vault hewn out of the soft and dry pumiceous rock and used, as was customary, for storing barrels and other paraphernalia. In the course of time, as more barrels accumulated, the grotto was excavated further and further into the entrails of the island.
A few miles N.E. of the town, I found, low down in the sandstone, a bed, a few inches in thickness, of a white, friable, harsh-feeling sediment, which adheres to the tongue, is of easy fusibility, and of little specific gravity; examined under the microscope, it is seen to be pumiceous tuff, formed of broken transparent crystals.
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