RECORD: Darwin, C. R. Geological diary: Abrolhos Islands. (3.1832) CUL-DAR32.49-50 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)
REVISION HISTORY: Transcribed from the microfilm by Kees Rookmaaker, corrections and editing by John van Wyhe 5.2010, further corrections by Gordon Chancellor, corrected against the manuscript by van Wyhe 7.2010. RN3
NOTE: This document, part of the largest scientific document composed by Darwin during the voyage of the Beagle, is written mostly in ink. Marginal notes are here integrated into the text.
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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.
1832 March 29th Abrolhos Islands
This group consists of
seven 5 islands & composed of Sandstone & Trap. — The inferior beds of sandstone 405. are fine grained. white, & cemented by ochreous particles. — In this there occurs the curved plate of Iron ore, which is so common in new red-sandstone in Shropshire. & pisiform balls arranged in planes. 18° S. — The sandstone, partly from this cause, weathers in a very extraordinary manner: the whole surface being honeycombed either by hexagonal or square or (a) circular cavities. 406 — On these rocks reposes a bed darker coloured indurated a luminous slate reposes 400 ... 404: does not effervesce with acids. super lamel crystaline rocks — When kept in a drawer my specimens exuded a fluid. — which was extremely acrid & rather bitter to the taste. — It did not affect Litmus or Turmeric paper. — was not precipitated by Mur: of Barytes: but largely so by Oxalate of Ammonia & (Nit of Silver?): was it this fluid Muriate of Lime?. — On this slaty rock there were was a superincumbent beds of coarse ferruginous gritty sandstone 390 ... 399: in places about 60 feet above level of the sea. — This formation dips at an angle of about 12° regular to N by W. — The accompanying rough chart will show its local situation. — The Trap is seen almost cutting through St Barbara .. formerly the SW island & I believe from its appearance the small NE one. — The rock is composed of a greenish grey Feldspar & small brilliant crystals 407 ... 409
[hand-coloured sketch map on tracing paper, apparently traced
from a book]
I. St. Barbara
Latitude 17°.57'.44" Sud
Longitude 41°.2'.9" Ouest Paris
Toises 100 200 300 400 500 1000
1832 March 29. Abrolhos Islands
of hornblende : it of considerable specific gravity & affect the needle. — In the SW it is divided into regular prismatic columns, in St Barbara has a cleavage running N 63° W. — In this latter place is covered by bed of Tufa. — On the Trap are large masses of an arborescent stalactite 410 411; I do not know of what composed but it is not Carb: of Lime. —
This rock overlies the sandstone; indeed the dip of the latter being N by W. the bare inspection of the chart is sufficient to show this. — The external shape of these islands accords very well with the strata that is dip to the same point. — in those in which the Trap predominates the same shape is preserved from which I suppose, that the originally horizontal beds of aqueous rocks were capped by the greenstone & then together with it heaved above the level of the ocean. — The occurrence of the narrow band of sandstone if S edge of the Trap at St Barbara & dipping towards it, is curious. as it separates the rock of this island from that of the SW one. — A hard modern conglomerate is forming on the coast, at high water mark, the same as I have noticed at St Jago & Fernando Noronha. — All rocks of course equally igneous origin, though most probably of far different dates. — This formation
1832 March 29th. — Abrolhos Islands
perhaps may be one of considerable extent. — Soundings are laid down extending for about 4 degrees E of the coast of Brazil (this is direction or strike of the strata) & about 2 in width. — The bottom must be extremely uneven, as each cast of the lead often gave most different depth. — Such notes as these can be of no use by themselves, yet may possibly be of a trifling assistance to anyone, who should accurately examine the geology of the Brazils.
I should think it certain that these beds. if occurring in Europe would be called Secondary: a class of formation very rare in S. America. —
1834. This opinion was solely owing to mineralogical character (of which I neither now or then know anything). — Why should it not belong to the Tertiary beds of Bahia. — Lyell P 81. V3.1 says that "laminated clay & sandstones form effects of lava. weather in most extraordinary honey-combed forms. Base of Etna. —
Baron Roussin2 states that all the outlying Islds are some times constituted as Castillo. — Figueira — Alcatrasses, Manoel Luiz. — But his descriptions of rock of this place are extremely vague, & not reconcilable with my specimens. —
1 Lyell 1833.
Baron Roussin...my specimens. —] added pencil.
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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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