957

1836

Terceira Azores

The Island of Terceira appears to consist of an elevated central part, which slopes down on all sides to the sea. — Both on the flanks and on the summit, there are scattered numerous volcanic cones, which yet partially retain their crateriform summits. — The streams of lava, which have proceeded from them can in some cases be traced*. — And the surface, especially in parts of the elevated central land, is covered by hummock & irregular depressions. — There more modern streams, when I saw them, chiefly consist of a blackish grey, cellular basaltic lava. — Specimens con (3912. 13. 14) show their nature. — Besides these streams, there are irregular ridges, & large hills chiefly composed, of a true, while or pale grey tractyle full of broken crystals of glossy felspar. brown lines & minute black points. precisely similar to the Trachyte of Ascension. — These lavas appeared the most ancient; & their circumstances present the old case of Trachytic nucleus, enveloped by more recent streams of a more basaltic nature. — I notice some lavas of a compact smooth texture, of a blackish colour, with crystals of glassy felspar, similar to some at James Is^d. —

[in margin in pencil] Count de Vargos. Denmark. has examined these Islds

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(a) * One great eruption of lava is on record but it happened many years since. —

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2

On the coast the streams of lava are separated from each other, by convoluted layers of tuff, some containing rounded pebbles & being perhaps subaqueous deposits, others consisting of only of consolidated volcanic ashes. M. Brazil, close to Angra, is a large & regular crater, (part of which the sea has destroyed) which appears entirely to consist of their layers of brown volcanic sandstone, quite similar to those at the Galapagos. — The extremities of the beds overlie the termination of the coast basaltic streams. — So that it is very modern. — Doubtless is only a dust volcano bursting out beneath water. — A little further to the Eastward, there is outlying island & some rock, which appear of a similar nature, but only a small segment of the crater now remains. — (Mem. the [pelit] of horizontal tufa, & inward dip of Webster)

At the Tour of Praya, it is on record, that after it was overthrown by an earthquake

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[pencil] I saw fragments of obsidian

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3

Terceira

the sea permanently gained on the land. — I saw part of a thick wall of a convent, which although lying on its side, seemed in its proper position, which is now bathed by the sea. — It is believed that it formerly stood above this level. — But the case does not prove anything with certainty. — During the last year several earthquakes have been felt; and for the space of some days a jet of steam issued from the perpendicular seaward face of M. Brazil, the sandstone crater already mentioned. In the centre of the island, there is a spot where boiling hot steam issues in considerable quantities. — There is a slight hollow, or little valley (like an ancient stone quarry.) the bottom of which is traversed by several wide fissures. the neighbouring rocks are all fine trachytes; & not particularly cellular. — The action of the steam, which is mingled with some acid vapour, has acted on these rocks in a most singular manner

960

& reduced them to day 3908 to 3911. The steam issues in nearly a dozen places through the hot & moist day. — This latter substance varies suddenly in its colour one part being snow white*, (a) another bright red, like Vermilion, and a third the two kinds marbled together. the Trachyte, which has been only partly acted on shows, a soft, semi-argillaceous base, with the glassy crystals only a little altered. the whole is mottled with minute red specks, which evidently is the oxidised iron. — In some fragments this substance is so abundant, that the Trachyte is the colour of a brick. — The sudden & complete variations of colour in the perfectly decomposed stone or clay is that must be owing I suppose to the iron being gradually dissolved in certain parts & again precipitated in others. — The water, from the surrounding banks when it rains, must penetrate the neighbo the fissure, at the bottom, for there is no exit to the hollow. — It is singular that the fissures appear as if the softened rock was degraded & then carried away. — But (a) where can it go? Is there a subterranean cavity?

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The figure of the depression of the ground, does not bring to my mind the idea of a subsidence, but rather of the washing away a ordinary degradation of the rocks. —
[pencil] wedge from crack. —
[ink] We visited St Michael. Both these islands in their general form, number of small crater, few great scattered mounds, [be] the logical nature of the rocks. — Sandstone craters. — Age, degree of activity, — general dimensions appear very closely to agree with the Galapagos Archipelago.
[pencil] No time Trachyte centre at Galapagos. —
* This white Porcelain clay is used for white washing the houses &c
[pencil] The degree of activity about equal to that of the Galapagos & Cape de Verds, ? Canary. With respect to age, of each enter degradation. — Sandwich Isd^s. —