RECORD: Darwin, C. R. Geological diary: Terceira (Azores). (9.1836) CUL-DAR38.957-960 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)
REVISION HISTORY: Transcribed by Kees Rookmaaker, corrections by John van Wyhe 9.2009. RN1
NOTE: This document, part of the largest scientific document composed by Darwin during the voyage of the Beagle, is written mostly in ink. Where pencil was used instead this is noted in the textual notes. Marginal notes are here integrated into the text. The paper size is uniform, 21 x 25.5 cm.
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Reproduced with the permission of the Syndics of Cambridge University Library and William Huxley Darwin.
See the introduction to the Geological Diary by Gordon Chancellor.
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 Isd. —
[margin][pencil] Count de Vargos. Denmark. has examined these Islds
(a) * One great emption of lava is on record but it happened many years since. —
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
[pencil] I saw fragments of obsidian
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
& 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?
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 Isds. —
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Citation: John van Wyhe, editor. 2002-. The Complete Work of Charles Darwin Online. (http://darwin-online.org.uk/)
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