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.


49

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1832 March 29th Abrolhos Islands

37

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

49v

[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
Ilots Abrolhos
Toises 100 200 300 400 500 1000

50

38

1832 March 29. Abrolhos Islands

38

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

39

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.

2 Roussin 1826-7.

Baron Roussin...my specimens. —] added pencil.


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