RECORD: Darwin, C. R. Geological diary: Cape of Good Hope (6.1836) CUL-DAR38.902-919 Transcribed by Kees Rookmaaker, edited by John van Wyhe. (Darwin Online, http://darwin-online.org.uk/).

REVISION HISTORY: Transcribed by Kees Rookmaaker from microfilm and the manuscript, corrections and editing by John van Wyhe and Gordon Chancellor 9.2009. RN1

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. The folios measure 20 x 26 cm and are numbered 1 to 18 by Darwin in the upper right corner. Folios numbered by archivist 902 to 919 on recto (upper right). Most verso pages are blank, except where noted.

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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.


902

1

1836 June
Cape of Good Hope

The few following observations apply to the mountainous peninsula, which forms Cape & Simons Bays & to a part of the mountains due east of the low sandy neck of that peninsula. —
The lower platform of the country consists of Granite. This generally raises to some hundred feet above the level of the sea, but in some parts does not at all appear. The granite is coarse grained & contains very large crystals of felspar; it is in many parts traversed by veins of ferruginous-siliceous granite; it contains balls of a dark color, which consist of an aggregation of minute scales of black mica forming imbedded in some tough basis; I saw in some blocks of decomposed granite, crystals of black schorl, which were placed, so as to radiale from a centre. 3654

Note where fragments were marked.

The granite is subject to extreme decomposition, & hence when protected, is covered by a great thickness of rock, reduced into the state of soil. — At the village of the Paarl, there are some extraordinary fine examples of loose balls of an enormous size, lying on the summits of the base

Note where fragments were marked.] added pencil.

902 verso

In my Rio de Janeiro paper there is a scrap with accounts by Dr Benza on hollows in granite made by stone which round & round
Daubuisson, M U p. 25 Balls of mica in granite
On recent formations of Hydrate of iron Vol II Daubuisson p 456 & 4761
Paalberg described Vol LXVIII p102
and Barrows travels p. 60
Dr Benza has remark on curious reaction in granite

1 Aubuisson de Voisins 1819.

903

2

Cape of Good Hope

mammiform hills of granite. Parallel & vertical fissures cros the mountains in directions at right angles to each other. These may now be seen of various widths, & it would appear that the boulders, great balls are only the remnants alone of original cubical masses. Besides the general description certain circumscribed patches of the granite give way yield to the weather, much more readily than the adjoining parts. As we see in some granites sphaerical masses projecting outwards from from processing a harder & slightly different structure, so here cavities exist on the sides of steep rocks [sketch] section; From the thinness of the overhanging lip, or front it appears certain, that no other cause than the quiet action of the weather has removed the central parts. A very large hollow, forming a cave, exists in the lower surface of one of the great balls on the Paarl. — This ball globular mass [sketch] is perhaps about 30 ft high, it rests on several points, within the

904

3

Cape of Good Hope

which is a smooth arched cave, frequented by cattle. On the sides of some steep masses, the granite is worn away, into extensive shallow cavities of irregular forms, which resemble the imperfect defective parts, of any mass of cast metal. — The Granite near Cape Town is traversed by several dikes, the greater number of which are parallel. They consist of trappean rocks, so much decomposed, that with the exception of their figure, & small side veins, there is little to point out, that they have not been fissures filled up from above. A sphaerical-concretionary structure in was visible in the wacke, & the undecomposed rock was present in some of the balls. — Near Simons Bay, there is a very small irregular vein of Basalt, & on one side of the Paarl there is a considerable mass of the same rock, but the relation of which, I did not see, from the covered up state of the lower part of the hill. — 3653 Cape Town itself is based on

905

4

1836
Cape of Good Hope

a clay slate formation. This rock is generally of a compact structure 3664 & of a dark blueish colour; it is associated with & passes irregularly into another variety, which has a glittering semi-crystalline fracture & appears to be of a felspathic nature. 3665: 66 Both varieties are The rock is frequently crossed by large veins of quartz;. — it contains a little mica. — The Lions rump is entirely composed of this formation; the weathering & subsequent alteration of the clay slate is there very remarkable. The hill to the depth, in some places, of 10 or 20 ft. appears encased, with a pale coloured sandstone 3667, but upon examination it is found, that all such varieties pass into the fine blue or slightly granular clay slate. The clay slate formation appears in patches in low country to the East of the Peninsula: P Robben Is is composed of it: it would appear to be the extremity of a great formation of the interior country; Dr A. Smith, who has lately returned from his most interesting expedition to beyond the Tropic, informs me

906

5

1836 Cape of Good Hope

that the great plains of the Karoo are thus constituted. — Dr Smith kindly conducted me to the junction of the clay slate & Granite, which is well known as described by Capt. B. Hall (Trans. Edinburgh R. Soc:) & Clark Abel. — The layers of the clay slate are everywhere directed in a NW & SE direction, the general dip is at an angle of 45˚ to the NE. In parts, however not far distant the dip is sometimes vertical & sometimes to the opposite point or SW. —

according to Dr Hall in high part very vertical

The line of junction of the Granite appears to cross the country in a line parallel to the cleavage of the slate. 3679: 80 The clay slate to the distance of a quarter of a mile appears is slightly affected:

At the distance the rock is &c &c

following the beach the slate is first observed to become of a compact nature & to possess an even, or slightly conchoidal fracture; parts of it t are semicrystalline & full of most minute particles of mica. — 3678 3677

according to Dr Hall in high part very vertical] added pencil.

At the distance the rock is &c &c] added pencil.

907

6

Cape of Good Hope

Within a hundred yards of the nearest Granite, the clay slate is changed into dark colored (felspathic?) compact rock full of minute scales of glittering mica 3675: 74; in some varieties which small collections of a soft white mineral, perhaps are preparatory to the formation of crystals of felspar 3673; another variety has a pale brown homogeneous base with the same granular collection of white matter, which in this case has been washed out 3676, by the action of the sea, & has left a curious honeycombed rocks. Close to the junction, the clay slate continues much in the same state, as a dark (felspathic?) rock (with a tinge of purple), with a compact or irregular fracture, & abounding with minute glittering scales of mica 3670: 71 72: —. the whole is full of small circular black spots, which gives to the rock a specido-granular appearance. — I have hitherto mentioned the junction, as if it existed as a defined line. it is really spread over a considerable space of nearly 200 yards. — The first appearance in the Granite is shown by small patches & short thin layers of the altered slate, imbedded at wide intervals in the coarse granite matrix

908

7

Cape of Good Hope

These gradually become more numerous, & although contorted & isolated generally retain traces of their original NW & SE direction. At last Presently the altered clay slate, yet traversed by veins of granite & including irregular masses, of granite, becomes the prevalent rock, & finally no trace of of that rock remains the injected matter is to be found & then the clay-slate by degrees assumes its proper character. Nearly in the midst of this passage the junction there is a large irregular dike of a white granite 3681, th different from the ordinary kind & full of quartz veins. — I can only compare this very curious junction to the appearance of the union contact union of two very fluids of very different degrees of specific gravity, which although they may penetrate each other, for the time keep distinct. The forms of curvature were are however different from what would happen in such a case. — By following the ridge of the hill from the

909

8

Cape of Good Hope

Lions rump to the Head, a beautiful contact of the clay. slate & granite may be seen (pointed out to me by Dr Smith). The clay slate is here changed into a thinly laminated rock, composed of small brilliant scales of mica separated by layers of yellowish granular mineral, which I do not know whether it is quartz or felspar; the laminae are undulating. — 3668 69. — I did not see any of the clay slate formation far South of Cape town, but near Simons Bar, Mr Sulivan discovered in the granite angular fragments of a blackish finely laminated rock in the granite which probably originally proceeded from the clay slate formation. Sir J. Herschel showed me some a curious specimen of a somewhat similar nature, which he obtained from Hout's Bay.
Far the most conspicuous feature in this country is a great formation of sandstone; This stratified mass attains, a thickness, which I should suppose must be about 2000 ft. — The varieties of the stone are

910

9

Cape of Good Hope

numerous 3655: 56 57.; generally speaking it is tolerably fine grained, white or stained with ferruginous matter 3658 .. 63, composed of quartzose patches, which not infrequently are blended together by a siliceous cement 3643. The degree of hardness varies much, but in the last named varieties is very great. — Some strata are coloured dark red & even black by the quantity of ferruginous matter. Occasionally in all parts a few quartz pebbles are found, but these would appear to be far more abundant in the superior series: — near Simons bay on the summit of the mountain there were sandstones which passed into a quartzose breccia 3642. We may imagine that the increased coarseness in the materials of the sandstones in the upper strata, to be the result of the shoaling of the water & consequent greater power of transport in the currents of the former sea. — The sandstone, where the white quartz pebbles were

911

10

Cape of Good Hope

abundant, resembled, excepting in being more siliceous & harder, that of the Blue Mountains near Sydney. — The sandstone is commonly traversed by large white quartz veins, & their are large surfaces coated with regular crystals of that substance. — all aqueous action
The strata are occasionally separated by layers of ferruginous shales. — I did not see a trace of any organic remains. — Besides the regular planes of stratification which are numerous, there are others nearly vertical of cleavage; they are far from universal, & I do not think any one direction is prevalent. Frequently in a single stratum, there are oblique seams or layers; this was particularly conspicuous in a snow white & rather soft variety of sandstone. — The sandstone reposes in an undisturbed manner on the granite, & as naturally might be expected, according to Dr Smith, on the clay slate. — At the Lions Head, the rounded massive outline of the granite is singularly contrasted with the horizontal strata of the

all aqueous action] added pencil.

912

11

Cape of Good Hope

cap of sandstone. — We did not find any actual junction, but at the distance of a few feet the sandstone & red shales were in an weathered state. Near Simons Bay, a beautiful junction is displayed, on the side of one of the mountains. The Granite forms a steep sloping talus of about 700 ft high, which is capped by the a great & almost perpendicular wall of about 1500 ft of sandstone. — It is interesting to observe the junction of two great [word heavily deleted] & distinct formations; one can picture imagine beholds the state in which the bottom of a deep sea once existed below & can see the very point, on which & one is led to reflect on the some change of circumstances which first commenced the deposition accumulation of the upper new & superior mass of matter. — The line of junction was appeared straight; the granite was quite decomposed, but every crystal in its proper place — on the granite it was superimposed a

913

12

Cape of Good Hope

layer, an inch thick, of the recemented constituents of the granite 3639, 3640, above this about 6 inches of a granite sandstone, & then commenced the pure fine grained siliceous sandstone 3641, which with the small variations already mentioned, is continued upwards to a thickness of at least 1500 ft. In another spot close by, the disintegrated crystals of granite were united & mixed in a thin layer of a dark ed indurated shale, on which lay the common sandstone. — A structure of this shale appears general, close to the base of the sandstone, for it likewise occurred near Cape town. —
With respect to the elevations of the mountains, inspection of a map, shows that the general line which has formed the Peninsula, ran nearly N & S. & therefore intersected at a considerable angle the old line of the clay slate & Granite. Near Cape town, an extensive mass of strata retain their horizontal position, & give rise to the well

914

13

Cape of Good Hope

known form of the Table Mountain (3500 ft). Further south towards Simon's Bay, a small westerly dip is common. — In some parts of the line, the granite instead of rising several hundred ft above the level of the sea, does is entirely concealed, in those parts the mountains are lower & I noticed in one spot [small sketch in margin], the strata were arched of sandstone bent downwards from the upper higher to the lower portion. We may hence infer that the granite & superincumbent Lava sandstone have been together elevated into mountain chains, & subsequently together have suffered degradation; Hence arises the form as if hills (indeed of a platform) of granite had originally been capped with the sandstone. — The chain of mountains which terminates at C. Hanglip, runs parallel to this first described line; when entering False Bay, the hills nearest the coast showed a very considerable dip to the East. At Sir Lowry Cole's pass, which crosses a

915

14

Cape of Good Hope

continuation of the same line, planes of division which I believe to be those of stratification, were nearly vertical. The sandstone was here based on a fine grained granite of a very different variety from that near Cape Town. —
It will be seen in any map of the colony that towards the southern shore, ranges of mountains run continuously for a long distance in an E & W course, in the whole country South of the French Hoeck Pass towards Genadenal, the structure of the mountains, probably from the intersection of these lines, appears singularly confused. —
To the SE of the French Hoeck pass, there are several short parallel lines of mountains, the strata of which dip to the ESE or SE. at angle about 20˚ or 30˚: These short lines escarpments sink towards their southern extremities by a succession of faults [sketch in margin]. — The French Hoeck pass (from the low country of the coast to the Eastward) crosses a part of one of these first & then E & Westerly range, intercepted between two short N & S lines. —

916

15

Cape of Good Hope

The strata dip to the South at angle of about 60˚, [sketch in margin] but have their superior extremities arched & flattened. — At the culminent part of the pass, the strata may be seen dipping to the SE; & from that they continue dipping more easterly, till at last they dip East & so form part of one of the North & South short lines, — If this observation, which was merely made in riding past, is correct, it is a curious structure, to trace have a gradually altering dip. round an elbow, from one system of mountains to another running at right angles to it. — Finally I may observe, that the structure of the Table Mountain, consisting of a great mass of horizontal strata, elevated to a height of between 3 & 4000 ft. is, when contrasted with the other chains of mountains, as remarkable a feature in a geological point of view, as it is a splendid feature in the landscape scenery of Cape Town. —

917

16

Cape of Good Hope

On each side of the low sandy neck of the peninsula, at the base of the mountains, there is a considerably extended superficial formation of a highly ferruginous stone, which is either arenaceous or aluminous. — in the latter case the structure is rubbly 3647: 48, 3651, the whole stone consisting of small irregular concrectionary balls so as to render it impossible to break off a large fragment. When the stone is ferruginous-arenaceous 3649:50, the stone is more compact solid, but is full of irregular cavities, containing, (where a fragment is first broken) loose sand; hence the external surface is honeycombed & of singular figures. — Mr Sulivan discovered high up in the mountains near Simons Bay, a small formation of this substance, bordering each side of a little stream or valley. — A fragment when first broken, showed, around each little ball of sand, a compact stone, so charged with iron as to possess a metallic lustre 3644: 45: 46, — In the stone angular fragments of quartz & sandstone were

918

17

Cape of Good Hope

imbedded. In this locality it does not appear improbable, that infiltration from ferruginous springs may have cemented the superficial sand. Dr Smith informs me, that tw all the mineral waters in the colony contain large quantities of Iron in solution. — In the level & extended flats, it appears to me scarcely possible, that the whole surface should have been acted upon by infiltration, when once raised into dry land, but I rather look at it rather as a subaqueous deposit. These ferruginous stones appear frequently associated with a yellow sandy clay: near Simons bay, a cliff of about 20 ft was composed of such a substance, it contained grains of quartz, & appeared to originate from decomposed granite, but not in situ; this substance was capped by about five feet of the rubbly ferruginous aluminous stone 3647: 48, which showed traces of horizontal stratification. In the lowest part, it first appeared in the granitic clay, as distinct small concretions of

919

18

Cape of Good Hope

irregular figures; This formation at nearly the same height fringed the shore for some distance. — I am inclined to believe the most origin of this formation, must have been subaqueous & if the superior matter is the result of infiltration, it has been a subaqueous process probably of no very ancient date. — In the low neck of the peninsula, (which has been several times mentioned,) the whole surface is composed of sand, with the exception in some parts of a calcareo, aluminous, arenaceous rock 3652; This resembles the stone, so frequently described by me, by the name of Tosca rock; the lower part has a white earthy fracture, whilst the superficial layer to an inch in depth, is compact & stalactiform. Near to Cape Town & to Simons Bay, there are some hillocks of sand, in which branches & trunks of trees are incrusted, with calcareous sand. The central part is either black & charred, or has been removed & then replaced by a cast of calcareous sand. (a) The phenomenon is seen [This sentence appears to break off. It is not continued on the verso.]

919 verso

(a) on a very poor scale as compared to that at K. George's Sound. The coincidence of these casts, the calcareous sand, & the concretionary ferruginous sandstones, likewise reminded me of that part of Australia. The origin of the calcareous matter with the sand of this country, may possibly be that indicated by Perron, namely the decomposition of the marine shells cast up by the sea, together with the sand.


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