RECORD: Darwin, C. R. Geological diary: Bahia. CUL-DAR32.3-8 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 and Gordon Chancellor 4.2010, corrected against the manuscript by van Wyhe 7.2010. RN4
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.
The granitic rocks consist essentially of gneiss; of this some is coarse grained & well characterized, but the 3838 greater part is highly felspathic: & frequently by losing its mica & quartz & gaining Hornblende & change colour of feldspar passes into a primitive greenstone.—
See specimens 3841-44
Allude to two kinds of incrustations on rocks of Ascension.
Feldspar in balls
Certain seams are highly micaceous & some few contain a
considerable portion of chlorite. — In one spot I met with nodular concretions
of garnets mingled with scales of mica. 3840 [sketch]
not infrequently these granitic rocks weather into these
forms which are common to trap rocks.
Mr Horners case1
In all [force] is [impressed]
singular peculiar modification of this structure presents
ar one part of the sea-beach a singular appearance.
A fine grained syenitic granite has its whole superficies drilled
with cylindrical holes, the bottoms of which are smoothly rounded. — These
holes generally have a
general diameter of between 10 1/2 & 3 inches.
& a depth of about twice or even more than these dimensions. Their position
is vertical & although separate one from the other, the partitions are
1 Horner and Brewster 1836. A note in Volcanic islands p. 54 Journal of researches 2d edn p. 10 reads:
Mr. Horner and Sir David Brewster have described (Philosophical Transactions, 1836, p. 65) a singular "artificial substance, resembling shell." It is deposited in fine, transparent, highly polished, brown-coloured laminaæ, possessing peculiar optical properties, on the inside of a vessel, in which cloth, first prepared with glue and then with lime, is made to revolve rapidly in water. It is much softer, more transparent, and contains more animal matter, than the natural incrustation at Ascension; but we here again see, the strong tendency which carbonate of lime and animal matter evince to form a solid substance allied to shell.
& change colour of feldspar] added pencil.
] added pencil.
(Pegmatite)] added pencil.
Allude to two kinds of incrustations on rocks of Ascension.] added pencil.
Feldspar in balls] added pencil.
on Ascension] added pencil.
Mr Horners case] added pencil.
In all [force] is [impressed]] added pencil.
St Pauls?] added pencil.
thicker, than the width necessary for strength. — This shelf
of rock being within reach of the tide,
each all of these little egg
cups stand full of clear water. — The resemblance is very close with some
varieties of honey combed sandstone. — On breaking a fragment an orbicular
structure is faintly marked, by curvilinear rounded ferruginous lines
of division. —
Dike & granite fragments
commonly is not always laminated but assumes compact & mended forms; yet the constituent minerals are arranged in lines which These follow no very determinate direction, but are curvilinear, & frequently form parts of great irregular circles. With all these irregularities, the laminae, where they can be traced, are may be said to run in a NE by N & SW by S [ illeg] line, more frequently than in any other direction. Neither the angle or point of dip follow any sort of rule; indeed the laminae are [ illeg] sometimes nearly horizontal, & then. dip away towards all sides at a considerable inclination. — I must remark
(compostion...cleavage] added pencil.
that the NE by N. [
to] direction is common parallel to the line of coast to the northward. —
The gneiss is traversed by dikes of red coarse grained granite,
as well as by irregular masses the latter appearing to blend with the surrounding
rock, whilst the former keep quite distinct. —
a few old & new threads like [illeg] to 48 [inch] width —
Hornblend not [Talcite]
Towards the southern end of the peninsula, on which the city stands, there occur numerous
well characterized dikes. of a black heavy basaltic stone. —
This is either has a compact or a granular base of Hornblende (?) & feldspar. 3833 to 3837 (
which the latter being sometimes greenish) which very commonly contains minute elongated cysts of glassy felspar. These dikes are well characterized.
[calculation] 45 [-] 11 [=] 34
Angular fragments of the granitic rocks are occasionally embedded. — These dikes are well characterized + they vary in width from a few inches to 6 & 10 ft
wide which latter dimensions are the most common. they frequently send forth tapering veins, whose which come run nearly parallel to the main dike. —
In several places,
I angular irregular masses of dark coloured rock are surrounded on all sides by the gneiss. — They consist of a granular
a few old & new threads like [illeg] to 48 [inch] width —] added pencil.
Hornblend not [Talcite]] added pencil.
mixture of hornblende & felspar, with a few scales of mica. 3830: 31 — I have represented the figures of some of these, appearing as if a dike had been broken up. In one case the cleavage of the gneiss appeared to fold round the fragment. This last circumstance occurred also with a considerable rounded mass of a similar granular rock (containing green crystals) 3832, which had the form of an injected body of igneous matter. —
It is certain in these cases the granitic rock must have been
nearly fluid, thus to have
embedded infolded extraneous bodies
؟ Have rocks which were traversed by dikes been metamorphosed & converted into gneiss
On the other hand; it must have been hard when fissured to
receive the true dikes, yet the compositions, excepting in the degree of crystallization
of the latter igneous rocks & the fragments do not widely differ. (?)
Moreover, I found one narrow dike (3836) of the
fine black fine grained
porphyry. which had been broken by faults in a most extraordinary manner:
one part seemed to be divided from another by a mass of Syenite coarse grained
granite: again on one side wall side of the dike angular
[sketch in margin]
؟ Have rocks...into gneiss] added pencil.
irregular pieces of granite projected within the lines of the
parallel walls; & a corresponding quantity of the trap rock was disseminated
& blended with the surrounding stone. — On each side to the distance of
some yards, small curved threads of the same dark stone intertwine, like ++
++ the finest cirrhi comae in the blue vault of the sky.
the thinnest veins
in the bodies called septaria, with the layers of gneiss; of these a few
can may be traced to the dike. — I can only imagine, that a substance
fluid as water being [insoluble] injected into such minute
fissures. — + Add this bracket at bottom of page (The intimate manner, in
which the two rocks are blended, would leave me to suppose, that both since
they came in contact sometimes had been semi-fluid) — I must observe that
these threads intersected some of the red granite veins, whereas these latter
intersected in another place intersected the embedded angular fragments.
+ I do not know whether the relation is constant. Nor have no I any
reason however for supposing all the red granite contemporaneous; An
examination of the mineralogical nature of the dikes & fragments will
settle the question of this reciprocal position. = (+)
The whole neighbouring country around Bahia, when viewed from a distance appears like a very level plain of about 300 ft elevation. At a few points near the sea beach, a level terrace of about 20 ft height may traced. The plain declines a little towards the sea, but is every where terminated by a steep wooded cliff: — The whole country moreover is intersected by very numerous, winding flat-bottomed valleys, which although not absolutely wide, are so when compared to the
water small size of the rivulet which they conduct. In one place a hill, which once formed part of the plain, was isolated by one of these low valleys, which no close to the coast branched off into two & so enclosed it. —
[sketch in margin] valley Sea
I need scarcely add after this description, that the structure of the country precisely resembles that of the plains of Patagonia, or any other soft formation which during gradual elevation has been modelled by the sea. In examining the cause of this occurrence, which is sufficiently remarkable in a granitic land, we find it partly owing to an extreme facility in decomposition & partly to the superficies being smoothed over with disintegrated matter. —
Unaltered rock is but rarely met with excepting on the sea beach; in the cliffs, it is seen 3845, gradually, but unequally to pass into a clayey mass.
in which however Higher up the constituent parts are distinguished with less facility 3846 3847 although yet the cleavage & occasional veins of quartz attest the rock is in situ. This mass is succeeded by a thick bed of very bright red clay argillaceous soil 3848 3853 which in its lower parts cannot easily be separated from the disintegrated rock in situ, but in its upper is full of partly rounded fragments of quartz & granite. This red bed forms the universal covering is often capped by another of pale coloured earth; & the parting seam generally containing some with more pebbles. — These two masses compose the superficial covering of the whole country; the red bed does not appear to be entirely conformable to the present form outline of the land, but the other has more of the nature of an ordinary alluvium, produced by the action of the weather. — I can entertain no doubt, but that the sea by modifying the decomposed gneiss
has deposited the red bed, & that the same power, during a gradual elevation of the land excavated the flat bottom valleys. — We shall presently show that the
coast shores to the height of about 20 ft & in some places to 40 ft is coated by tertiary deposits, which attest this elevation. And we may observe that to the southward the land coast is intersected by creeks & by large bays studded with one islands, which form of land must once have occurred in this immediate part. There is however a considerable difficulty in imagining under what circumstances the red gritty clay was deposited; at the present day [ illeg] the [ illeg] line of coast is either composed of bold rocky cliff or white sand beaches. — The tertiary formations likewise consist of hard sandstone or coarse conglomerates. — Yet It must however be allowed that the circumstances were very different, when a great level tract first emerged, & the land at intervals continued to rise, from the present time, when a strongly marked lines of cliffs line of cliff attests for how long a period the sea has washed its base. —
Earthquakes are quite unknown, & therefore we may presume that no changes of level are now in progress. The fissure, filled with sedimentary matter, bespeak a time
We will here group together a few observations on the decomposed granites of Brazil.
Rio observations. — Prince. Maximilian.1 at Bahia I believe the chief difference with the decomposed granites of other countries, which I have seen, is its more argillaceous nature, which might well arise. —
Pennant Spix & Martius2 quote the contrariety of opinions. — Examine Eschwege3 interesting formation, curious minerals fractured state, gold, crystallized, diamonds. — great extent. (Pernambuco Dr Webster).4 — It is clear something very remarkable in this formation. — The only palpable difference in subaqueous decomposition, which was rendered proba certain at Bahia. — We know from C. of Good Hope, that granite at the bottom of a deep ocean may be quite decomposed. —
Alluvium] added pencil.
The whole line of coast is skirted with patches of sedimentary rocks. These consist either of hard conglomerate consisting of round large fragments of the surrounding granite rocks, or of
ferruginous sandstones, some ferruginous or of others white hard sandstone & containing fragments of marine shells. These This latter stone in several places on the beach from sloping a breakwater, sloping to seaward; but it is separated from the cliffs of the shore, by a low cliff wall composed of the basset edges. —
A level of sea at high water
B do at low do.
Is this caused by the recoil of the wave at high water, when they strike against the base of the cliff. — These shelves are divided by straight fissures, which have in many places divided the whole mass, so as to allow parts to subside. It is difficult to tell how much of these rocks
are owe their origin to causes yet in action. — In one spot I saw a thick mass of coarse conglomerate, but just removed above high water, which capped some points of granite.
NB. This might have been formed by sand filling up the chasm & being again removed by same causes which produced the fissures)
Since its consolidation parts of
this last granite a latter rock have been removed, hence the stratum of conglomerate forms a flat archway.
A. The sea at high water. —
I conceive there must have been a change of level to have produced this effect. — The imperfect terraces which I have said in a few parts, may be seen to skirt the coast consist of ferruginous
& [illeg] sandstone, loose sand, & the disintegrated granitic soil. —
At Bonfin the strata attain the greatest thickness being there in parts at least 40 ft. — Conglomerates abundant coarse very hard. — The rest of rocks. lie by the side of these; consist of thin layers [ranging] in nature from a grey fine grained [
illeg] sandstone to clay slate, which is either pale grey, blue or even black. — A vertical section shows extreme contortion & fracture, in strata. — a horizontal section shows curvilinear layers, which appear to fold round certain points below, but dip to seaward. —
during the process of consolidation
The real cause no doubt has been [illeg] subsidence. — The most remarkable circumstance in these sandy shales, are irregular, even angular concretions of a hard, fine grained non calc: grey sandstone 3849: 50; These sometimes unite & form irregular strata: when distinct they are of all sizes, from a few inches to several yards in dimensions. —
NB. Some few of the concretions appear
contain one calcareous & even of a crystalline structure.
The surface commonly is very rough; and in the weathered fragments are thickly coated over with distinct small perfect rocks of Carb. of Lime. — I am not sure whether
these latter surfaces. were the original ones, or result from
concretions fractures in the lines of former fissures. — In the lower part of the cliff several of the concretions contain numerous elongated shells; & I found one concretion mass of stone with a more obtuse kind. 3851 No doubt, those I procured last year came from same site. — I saw none except in these concretions. at the present day at high water these stones are covered & the strata dip to seaward. So the Balani are now adhering to the front — water shells. — These strata are covered by about 40 ft thick of similar depositions. — At the present day. Neither the boundaries of a lake, or the channel of a large fresh water stream entering the [valley] can be reconciled to the present form of the land. The shells may have been washed into their present position, but I should not think it very probable, as they appear to be grouped in families. +
+ most likely they lived at the point where river joined the bay; agrees with greater thickness of deposits & coaly matter. — We are compelled to this conclusion because the facts
The facts are
known this very quite insufficient to prove a subsidence; without we think Baron Roussins soundings give probability to the idea of a formerly more extended formation. —
Finally I must mention that a French gentleman presented me with a large Univalve (Marine ?) shell, embedded in a pale-coloured limestone, which came from a great distance in the interior near R. S. Francisco. 3852 — This would indicate some formation probably of a tertiary epoch. — He had heard also of a large fossil bone having been found in same neighbourhood. —
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