RECORD: Darwin, C. R. Geological diary: Bahia Brazil. CUL-DAR32.9-14 Transcribed by Kees Rookmaaker, edited by John van Wyhe. (Darwin Online, http://darwin-online.org.uk/).

REVISION HISTORY: Transcribed from microfilm by Kees Rookmaaker, corrections and edited by John van Wyhe 4-5.2010, corrected against the manuscript by van Wyhe 7.2010. RN2

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.

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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.


9

(1 Bahia Brazil

Towards the extremity of the point of land, on which stands the city of Bahia in Brazil stands, I observed some rocks, belonging to the grand granitic formation, coated overlaid with a bright glossy coating. — The point where this recurred was where a small fresh-water stream rivulet entered on the sea beach. — The colour of the coating is of a fine rich brown colour which sometimes passes into a black; & when seen over white quartz. is that of bright mahogany. Its composition appears ferruginous, it does not effervesce with acids; under the blowpipe, it stains borax of a pale yellow. — The layer is exceedingly [illeg] thin, not exceeding the 1/250th part of an inch, yet it forms perfectly envelope to the rounded masses of rock, which close to or within the bed of the stream, projects through the sand beach. These [laminated] rocks have The rock consists in this spot happens to be a primitive greenstone, which by assuming a laminated structure & large scales of mica passes into gneiss; the nature of the rock appears to have no effect on the coating; (a) some small pebbles which were fixed in a crevice were equally & (2) polished (1) with the solid rock mass. At some former period large fragments have been broken off, which probably served for the erection of a neighbouring house, now in ruins, but the

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(a) The rock is unaltered close to the beneath the layer

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2

new & rough surfaces have been since received a similar & continuous varnish. 3844 — Hand specimens fail to give a just idea of the appearance, of these brown burnished rocks 3841 ... : 43 showing which glitter in the rays of the sun. — The extent of surface thus covered is extremely limited space however in which they occur, is extremely limited namely from high water mark to about half the width of an ordinary sea-beach; & only a few feet on each side of in the line of the fresh water streamlet. — The polish gradually becomes thinner & dies away above the limit of the spray at the high water tide. the most favourable circumstances points for its production, are slight depressions or pits in those parts of the rocks, which lie within the bed of the rivulet but are daily exposed at high water, to the wash of a considerable surf. — d> The greater part of the fresh water in the vicinity is (probably from plummeting through the [decomposing] [formation] [chalybrate], in its [illeg], & deposits a slight a flocalent nut coloured matter in its bed (a). The position alone, of the coated rocks, would appear to indicate that the fresh water deposits the matter, & that the action of the surf consolidates & polishes it. — (b) In this latter respect there is a strong analogy with the circumstances which produce the

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(a) In this slowly trickling stream there are some masses of brown ferruginous sandstone & conglomerate of recent an origin, very recent, but I am not certain whether of the present day. —

(B). Note Would this rivulet, if it had precipitated its waters in a cascade have coated the basal rocks? I doubt it — In such a case. it must be remembered there would be no periodical action of unchanged water. —

(d) The most projecting corners are but very thinly varnished, & they appear as if a thicker layer was in the process of being gradually washed off. — This appearance would lead me to suppose that the process is constantly varying its productive [phenomenon] is subject to frequent variations in degree & exact renovations & decay

its productive [phenomenon]] added pencil.

renovations & decay] in pencil.

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(3 Bahia. Brazil

dark grey calcareous varnish at Ascension. From this analogy Must we believe that the ferruginous matter is deposited at times when the rocks are protected by an accidental covering of sand or shingle, or merely at the intervals of low water? — I rather incline to the former of these supposition because in one spot a patch of conglomerate still adhered to one side of a burnished point. —

But Again how does the mechanical action of the surf aggregate consolidate into so hard a substance the ferruginous matter? Does it merely approximate force into close contact particles already united deposited; or are much particles deposited in the mingled waters of the stream & sea, & by the violence of the flow must we not rather believe that some ferruginous matter being deposited in the mingled waters of the stream & sea is by the [illeg] precipitated either from the vertical disengagement (from the violent impaction) of Carb. acid [gas] or by some other mean cause means. — However this may be it throws light on the production of conglomerates with a ferruginous matrix on exposed sea beaches. —

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12

(4

There are some phenomena intimately connected with this subject, which have been ably discussed by Humboldt in the Personal Narrative. Vol V Ch P I Page 18.1 In certain parts of course of [illeg] Near to the cataracts of the Oroonocko, at those of Syene on the Nile. & at the "yellalas" (rapids or shoals) of the R. Congo or Zaire, granite rocks are found encrusted with a layer of a a smooth glossy black layer (as if coated with black lead) which on analysis is found to consist of the oxides of iron & manganese. — The (?) layer is 0.03 003 of inch of a line of inch thick. — In this latter respect, in the colour & composition there is a marked difference [2 words illeg] & likewise the phenomenon occurs in a much larger scale. In the nature of the rock (. There evidently however is a close connection in the two covers in the rock is with my case; yet, besides the external general aspect which must be somewhat similar, there are several points of resemblance. In the three great rivers, the phenomenon occurs only where the current is broken & rapid. At the Oroonocko, the Indians say "that the rocks are black, only where the waters are white" Humboldt remarks they ought perhaps to add "where the

1 Humboldt 1819-1829, vol. 5, p. 18.

[marginal sketch in pencil] .004

003 of inch] inserted pencil.

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13

5

Bahia

waters acquire great swiftness & strike with force against the rocks of the banks"1 Humboldt also adds states that "all these phenomena of coloration have hitherto appeared in the torrid zone only, in rivers that have periodical overflowings"2 & in the Orinoca. the layer is said to occur in the parts periodically washed by the waters. — It In my case the it is easy to believe that the tidal action corresponds to the periodical overflowings. & the action of the surf. to that of a rapid or a cataract. — (a> In all the cases the rocks are granitic, but as such formations contain such distinct minerals as quartz. felspar. mica & Hornblende. I cannot believe their composition can have produce any effect. — Probably all that is required is a solid surface not subject to rapid decomposition. There is another coincidence in the intertropical climate, which likewise is common to the calcareous varnish at Ascension (? at Serpentine at St Pauls). but it Berzelius, at the request of Humboldt

1 Humboldt 1819-1829, vol. 5, p. 24.

2 Humboldt 1819-1829, vol. 5, p. 21.

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(a) We may even venture to conjecture that the comparative slowness of the stream at one time of the year, may allow of deposition. & that during another the waters may burnish 2 & consolidate 1 what such matter.

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examined some encrusted specimens from Syene. "this celebrated chemist thinks, that the rivers do not take up these oxides from the soil over which they flow, but denude them from subterraneous sources & deposit them on the rocks in the manner of cementation by the action of particular affinities, perhaps by that of the potash of the felspar."1 -/- It may be asked how comes it, then that while quartzy pebbles (a) are (2) equally or even more (according to [Humb] varnished with rocks the felspathic rocks? — With respect to the source of the ferruginous matter at Bahia, I think there can be no doubt it is owing to the percolation of rain water through the bright red clayey soil, which has resulted from the disintegrated gneiss. —

++ Humboldt adds. "cementation seems to explain why the crusts augment so little in thickness."2

1 Humboldt 1819-1829, vol. 5, p. 24.

2 Humboldt 1819-1829, vol. 5, p. 24.

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(a) in my case. & the quartzose parts in the granites of the Orinoco are varnished equally &c &c


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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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