RECORD: Darwin, C. R. Geological diary: Bahia. (2-3.1832) CUL-DAR32.41-48 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 against the manuscript by John van Wyhe 7.2010. Correction by Gordon Chancellor 11.2012 RN2

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.


41

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Bahia. Febr 29th .... March 17th          29)
1832

The town of Bahia is situated on the side of a promontory, which runs about NE & SW & forms the Eastern part of the bay of All Saints. — Owing to my being lame I was only able to examine a small part of the adjoining country. —

Pegmatite The peninsula is composed of Gneiss & beds of Trap. these rocks vary in their characters & pass into each other. —

312. 313 314

(x)

310

311 X

315. 316 343

317


The Gneiss either has the quartz & hornblende arranged in lines in a red crystalline feldspar, with but little mica; or it has the more ordinary appearance of all the constituent parts being in planes. in this latter case it is more slaty & decomposes more readily. — The trap is of a grey colour, very brilliant fracture & fine grained: — When it contains Mica it passes into the gneiss, the transition being marked by the Feldspar becoming reddish & the parts grouping themselves into lines. — This rock is best seen at southern end of the town, but generally it is not nearly so frequent as the gneiss, with which it alternates: These beds are frequently crossed or interlaced by veins or dykes of a reddish gneiss or syenite. — The rocks of which this is composed is often oftentimes fine grained; but generally has the Feldspar in large red crystals. — I observed in one place crystalline Carb: of Lime & traces of Chlorite. — These veins had the same form as Basaltic ones, & immediately gave

41 verso

(a)
The junction of the veins was singularly well defined. —

(x)

344

X

In one place I found the gneiss composed a fine grained mixture of Hornblende mica & Feldspar. — The feldspar being collected into granular ball, gave to the rock at a distance as conglomerate appearance.

(b)

323. 324

The gneiss surrounding the Hornblende in Fig II contains numerous large crystals of hornblende. — Can it be supposed that the gneiss obtained this larger quantity from the imbedded rock? or is it merely an accidental circumstance. —
  Humboldt mentions that the gneiss-granite about the Orinoco is characterized by the presence of such large crystals. —

41v

Fig I
[sketch] A A B B

Fig II
[sketch] A A B B

A. Large bed of Gneiss: B. imbedded portions of Hornblende rock.

Bahia

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Bahia. Febr 29th .... March 17th 30

(a)

318

320. 321 322

the idea of being injected from below: These veins were well contrasted by others. which of an earthy. calcareous nature, which ran in straight lines & were evidently fissures posteriorly filled up. — The most curious phenomenon I observed in these formations was the entanglement of angular pieces with very defined edges of Hornblendic rock in the gneiss. — This appearance I have represented in Fig. I & II. — In I a mass of the rock is cracked

(b)

 

 

319

& only partially separated by the gneiss. — This hornblendic rock is of a black colour finely crystallized (with a Feldspathic base?) & melts easily into a black enamel. — Again this rock occurs in dykes, varying in thickness from 4 inches to as many yards. — I could trace the small veins to the thicker, & from the latter were sent off pointed wedge shaped masses into the adjoining gneiss: The hornblende only differed from the entangled pieces in not being so much crystallized. — The double case of the Hornblendic rock penetrating the surrounding mass & chiefly in angular pieces being caught up by the gneiss, appears to me very curious. — If these phenomena are explained by agency of heat, we must suppose the melted masses of the two rocks to have been poured forth at nearly the same time, & at that in different spots

42 verso

(a) And after both were cooled & hardened they were traversed by the veins of red gneiss.
(b) Again "M Lechenault de la tour collected in the bars of the river Mana in French Guyana the same gneiss granites (with a little amphibol) which I observed three hundred leagues more to the west near the confluence of the Oroonooko & the Guaviari" Humboldt. Pers. Na. Vol: VI. P 604. —1
  Who is M. Lechenault de la Tour. has he written? —2

1 Humboldt 1819-1829, vol. 6, p. 604 note.

2 Louis Théodore Leschenault de la Tour (1773-1826), French botanist who travelled in Brazil, Guiana and Surinam in 1823-24. No publications have been identified.

Who…written?—] in pencil.

43

1832 (31
Bahia. Feb 29 .... March 17 31
Cleavage either one of the two to have hardened first & subsequently to have been broken up or penetrated by the other. —
(a) The beds of Gneiss & trap dip at about an angle of 70° to NW by W: but at extremity of penin promontory more northerly, viz. NNW. — The strike of the strata is in consequence nearly NE. which accords, as far as I was enabled to observe with the range of lower hillocks, which compose the country. —

45 [-] 11 [=] 34

!

(b)

 

 

As far as the eye could reach the dip of beds was most regular. — Humboldt mentions that in gneiss-granitic formations in Columbia the dip for 300 hundred miles is exactly N 50° W: Now this agrees most curiously with the greater number of my observations. — From this fact & from the rocks of Bahia well agreeing in their mineralogical character with those of Columbia, I should consider them, although separated by the great distance of 1380 miles, as the same formation. If this remarkable continuity was more satisfactorily proved, it would be a fine instance for those who attribute the present state of the world more to great causes at distinct epochs, than to a succession of smaller ones. —

 

Most of the my observations were made on the natural section presented by the sea-coast:

The country surrounding Bahia is elevated

Cleavage] added pencil in margin.

45...34 !] added pencil in margin.

43 verso

Alluvium =

Alluvium =] added pencil.

44

(32

1832

Bahia. Feb29th .... March 17th

32

between two & three hundred feet above the level of the sea: it is composed of a succession of small hills & valleys formed by the agency of water. —

The soil is of a singularly bright red colour & very fertile: nothing can exceed in beauty the wild luxuriance of the vegetation. — These hills consist of a mass of decomposing gneiss. — it is either in the state of a red clay, not effervescing with acids; or a confused mass of crystals of Hornblende Quarts & mica 326...329. — in which however a regular dip to NW by W is generally perceptible (maybe about 70°):

On this there is a thick bed (15 feet) of soil, containing a few pebbles. — I am at a loss to understand how such a thickness of rock could undergo so complete an alteration & yet the vestiges of strata remain undisturbed. — In one instance I traced a vein of quartz from the clay into the unaltered gneiss. 330 —

I now pass on to describe some scattered remains of a formation far more modern than the one already mentioned. — It is best seen at Bonfin, a village a few miles north of Bahia & situated on a neck of lands which projects from the bank on which the city is built. — The rocks here, so frequently change, that the best plan will be to describe them as they occur in going round this promontory or neck. —

44 verso

(a) I could perceive no signs of organic remains in any of these clay or sandstone beds. —

(b) It is said a coal mine was attempted to be worked near Bahia: Spix Vol I P 2801 March 1833

Consult Latter parts volumes of Spix & Von Martius

1 Spix and Martius 1824.

Consult…Martius] added pencil.

44v

Fig 3
[sketch] A B A

Bahia A. Blue slaty clay : B Greyish green sandstone

hand-coloured sketch, captions in pencil. Verso is blank.

45

33

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Bahia. Feb 29th . March 17th

33

At the extremity is a considerable thickness of conglomerate, formed of large boulders of Gneiss. Trap, & Syenite cemented together by base of minute broken crystals. — On the top of this is situated a small fort: these beds alternate with a ferruginous sandstones & a green slaty clay. — They dip at 5° to N by E. — And this may be considered as the dip for all the following beds. — Proceeding onwards the conglomerate dies away & is succeeded by the clay beds. these in places are covered by a hard fine grained siliceous sandstone, not effervescing with acids 369...372 & containing scales of Mica. — In one spot I found beneath this latter rock a thin bed (1/3 of inch thick) of coal 374. — colour fine black; hard; burnt with bright flame & bituminous small. — (b)

Yet further we found the sandstone, often times soft becoming more abundant. — 373 There again (a) the slatey clay is predominant; In Fig 3I have drawn a section, where the strata present a curious formation. — The cliff is about 40 feet high & consists of a bluish clay; towards the lower parts there are regular beds of a sandstone 375 383; but in the upper, these are curiously contorted: This cannot be owing to subsequent violence but to some cause acting during their deposition. — In sandstone there occur large globes of clayed sandstone. & likewise

45 verso

(a) I only discovered this two days before we sailed

(b) The great neck of land on which Bahia is built

46

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Bahia. Feb 29th . March 17th

34

large masses of (Clay Iron stone?). 377 — All these rest on a green slaty clay sandstone containing mica & very curiously bedded. — A horizontal section 376 (produced by the tide) presents concentric strata dipping from one centre: This is owing to the slaty clay being arranged in enormous balls about 40 or 50 yards in diameter. The beach has from this cause a most singular appearance. —

Nearly at the base of the neck of land a white soft sandstone forms large beds: this is veins like wood with ferruginous markings. —

And finally a small bed of a breccia with marine shells; this dips to N by W; on I shall have occasion to describe presently a similar formation. —

On the beach in several places large masses were lying of a blackish argillaceous limestone, abounding in fresh-water shells. 378...382 — such as Planorbis. L Melanic. Lymnaea. — (Nerita Cyclas?) (a) Although I could not find the rock in situ (c) I can have no doubt but what it was near at hand. — from being free, that & the sandstone were used for rough architectural purposes. — I do not think it was brought as ballast. —

I found vestiges of a formation to similar to this one those at Bonfin, surrounding the promontory. (b) (b) the older rocks of which I have described.

46 verso

We must suppose, at least granite decomposed beneath the sea, —

(a) the clay contains the oxides of Manganese & Iron in nests. 336 — One great bed of this red clay much resembled the common covering of altered gneiss. — The veins of jaspar, solely pointed out the difference

(b) The beach here consists of a white & brilliant sand. — which is seen far off & affords to navigators a good feature to recognize the coast. —

(b) SSE of town of Bahia & across the promontory

(c) From the occurrence of Fresh & Salt water shells is it not probable that there must have been several elevations & depressions of the land ?? — It is by this latter phenomenon that I only am able to understand the sandstone dipping apparently under the Gneiss. —

We must…the sea] in pencil.

47

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35

1st. there are traces of a conglomerate of gneiss reposing on the strata of gneiss. —

2nd a considerable bed of a white & hard breccia formed of fragments of transparent quartz united by a calcareous base 337...341 342: it abounds with marine shells. Serpula. Corals. & mostly bivalves. — the bed is much fissured & nearly horizontal. it clearly reposes rests on the gneiss.

Conglomerate perhaps partly modern partly old

Being fine & of white & brilliant appearance it is used for architectural purposes. — Untill I found a similar formation at Bomfin I was inclined to think that this was a breccia formed since the present order of things existed. yes —

(b) 3d a highly ferruginous sandstone: which when not decomposed is formed of grains of quartzy united by specular Iron: through this there runs curved plates or veins of a compact sandstone even abounding with more ferruginous matter: The same fact occurs universally in the new red sandstone of Shropshire. —

As we proceed along the coast these sandstones 331, 332 increased in thickness & alternate with (a) beds of red & variegated clay. often containing boulders of gneiss: through these thick veins of a Jaspery rock run. 333, 334 —

(c) These narrow stripes of strata dip at a small angle to the North, that is from the sea: they extend only a few yards inland as the proximity of the older formations prove: in many places by this action of the waves that will soon nearly disappear

Conglomerate perhaps partly modern partly old] in pencil written over other entries.

Yes] added pencil.

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Bahia. Feb 29th .... March 17th

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In Baron Roussins chart1 soundings from twenty to fourty fathoms are laid down as extending eight or nine miles from the coast. — The bottom is described as "sable et vase verti compacte." As we have seen at Bonfin that the inferior beds inferior to the conglomerate & sandstones were green slaty clay. — is it not very probable that a similar formation extends at the bottom of the sea? —

Owing to the short time, during which I was able to walk, the two extreme points where I observed these formations were only 5 & 1/2 miles apart. — In all cases it had the appearance of being a broken off part of a greater formation. — what the limits & conditions of this might have been of course I am quite unable to say — I should think it certainly belonged to the class of Tertiary formations where the occurrence of beds containing both fresh water & marine shells has frequently been observed.

These tertiary formations are much the same as those described by Humboldt in Columbia: he also mentions a fresh-water calcareous tufa, but gives does not specify the organic remains, by which it may be recognized.

1 Roussin 1826-7.

48 verso

NB. In the Chanticleers Voyage1 nature of a Sandstone at Pernambuco is mentioned; It would be well to look again at Prince Maximilians book. —2

A. Boué. Journal de Geologie t.2d P. 205 talks of Tertiary formation of Bahia3

Put note to my description of Bar of Pernambuco4

1 Webster 1834.

2 Maximilian 1820.

3 Boué 1830.

4 See Darwin 1841, F266.

N.B….book.—] added pencil.

Put note…Pernambuco] added pencil.


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