RECORD: Darwin, C. R. Geological diary: Monte Video. (8 & 11.1832) CUL-DAR32.77-82 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 against the manuscript by John van Wyhe 6-7.2010, corrections by Gordon Chancellor 10.2011. 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.


77

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1832 August & November Monte Video 63

The country in the neighbourhead of the city is composed of Phyllade (Daubuisson) & Granitic rocks. — By the latter I mean Granite, Syenite, Gneiss, Mica slate; generally passing into each other & not often well characterised. — The two latter are most abundant; the town itself is built on a Gneiss in which the quartz & mica are in planes & surround Feldspar 852, not well crystallized. — Towards the East side, of it the rock is granite (a), or more properly Feldspar in large crystals, with the other ingredients sparingly distributed: in it; here it also frequently alternates & passes into Phyllade. — In short it is impossible to describe this formation any nearer, than by naming the three ingredient minerals, quartz mica & feldspar 650. 651 (granular felspar), which often most generally occur with a slaty structure. — At the mouth St Lucia I found some true granite with feldspar in small crystals. — in other places, Hyalomictite occurs in paps projecting above the soil. 661. 662. quartz granular with little silvery mica. — At Las Pichas, there is a large formation of beautiful Syenite: Feldspar in red crystals, decomposing rather easily. Hence the rock has assumed from degradation the appearance of Boulders: Some of the masses have withstood the action of the weather & assumed a rough polish. 854 853 — the stone would be very ornamental for architectural purposes.

Friction of sheep & other animals!!!

The commoner rocks. mica slate & Gneiss, as has been said frequently alternate with Phyllade

(granular felspar)] added pencil.

Friction of sheep & other animals!!!] added pencil.

77 verso

Is cleavage, (in so [untroubled] a country, very unusually vertical ??

When I return to M Video, Geologize Rat Island: the Mount East of the City: also measure barometrically the Mount.

(a) I saw east of the city veins of feldspar in large crystals

Also beds of shells at Mount & within city: It would be curious to see if rock in Rat island is Gneiss — change to Phyllade & from that to greenstone on Mount:

(b) Greenough P 78. — mentions that quartz in Mica slate sometimes appearing as bed in another place would be called a vein: & that quartz often is in contorted lines:1

(c) Streaks of quartz & hornblende slate occur in take the place of Mica slate & when show a tendency to become clay-slate. — Von Buch P 2362

(d) This is not quite accurate. (Novemb: 1833) the rock between bands of quartz very fine gneiss. —

1 Greenough 1819, pp. 77-8: "In the mica slate which occurs between Lough Mask and Lough Corrib, in Galway, the mica has a determinate direction, and its laminae are straight, but the intervening quarz is sometimes straight, sometimes curved, and varies so materially in its direction, that what in one spot would be supposed to be a conformable layer, exhibits in another all the characters of a vein."

2 Buch 1813.

Is cleavage...barometrically the Mount.] added pencil.

Also beds of shells...on Mount:] added pencil.

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& their cleavage & stratification is identical. The beds of the Phyllade vary in thickness from a few yards to more than a mile. The rock 652 ... 654 is soft, fissile, of a green tinge; the mica tending to pass into Chlorite as it does in others into Hornblende 655, 679. — Then becoming Hornblendic slate. — At Rat island a small mass of Phyllade, a few yards square, is enclosed on all sides by Mica slate & has the same cleavage with it. — it is most singular 648, 649 by the intricate interlacement of quartzose curved veins. — These veins directly unite with the surrounding rock. at first sight it would be imagined that the piece of Phyllade was entangled within & penetrated by the mica slate, during its (the latter) formation. — But the identity of cleavage of the two rocks proves them to be formed at the same time; must we not suppose then (b) that these veins are a chemical separation? (c) — To the East of the city, the passages between Phyllade & Mica gneiss slate might be observed in every state; in places (c) the quartz formed straight parallel bands & the intervening substance, not to be distinguished from the Phyllade (d). these plates formed a rock showing vertical cleavage in the direction usual in the country. — These bands of quartz were united with masses of siliceous rock which ran across the lines of cleavage. — It is difficult to understand their formation: if by deposition we must suppose that the alternate parallel layers

78 verso

(a) These infolding veins of quartz, reminded me of a formation of that rock at Holihead where even in a hand specimen, the rocks seems composed of curved plates. —

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1832 Monte Video 65

of silex & phyllade were deposited horizontally alternately under an expanse of water; but then how can we account for the mass of siliceous rock uniting with the quartz plates both above & beneath the Phyllade. Cleavage. Surely this shows a simultaneousness & not an alternation of action. — However this may be explained, the same cause produced the cleavage; & as this latter & the ?? rocks are generally the same over the neighbouring country, we may suppose it to be the universal one. For the reasons above given, I cannot believe it to be owing to deposition but rather to some molecular attraction between the particles governed by laws of which we know nothing. — Cleavage The cleavage in all these rocks is generally well defined (a) & runs E (or with generally a point North) & W; the dip is nearly vertical: But to the east of the city it is only 52° to the SSW. — This applies to the stratification; as shown by the function of the different strata. — Besides the line of cleavage, there occurs both

[bottom of page excised, located in CUL-DAR42.75]

p. 65 M. Video

in mica query slate & phyllade, but chiefly in the latter, smooth parallel planes of fracture: these cross those of cleavage & of dip to one of the points of direction. —

fissures{

This was generally towards the East & at angle from 10° ... 20°; thus forming steps on the edges of the plates of cleavage: —

These facts were chiefly notcied on the Southern W base of the Mount.

Cleavage] both instances added pencil.

79 verso

(a) The extreme points. where I observed the direction of the cleavage, are about 15 miles apart, on the coast in a E & W line. —

80

66 1832 Monte Video

In connection with this subject, it may be remarked, that if the beds of Phyllade & other rocks, which are now nearly vertical, were originally superimposed in horizontal strata; how strange it is, that they should have undergone such violence as to have passed through from 60 to 90° degrees of elevation; & yet the country to be, the nearly as tame & untroubled as the most modern formation on the southern shore of the Plata. —

It appears to me quite incredible that the crust of the globe should undergo such (imagined) fractures & yet neither mountains or valleys be formed. —

The Mount, from which the city of M: Video takes its name, is said to be 450 feet above the level of the sea. — it is by far the highest land in the country. — It is composed of Phyllade & the longer axis in the direction of the cleavage, viz. E & W ascending to the summit 656. 657 658. 680 681. 682, the slate gradually becomes harder, more compact & sonorous, with a slaty conchoidal fracture; at last it is converted into a slaty greenstone, & in some places (a) is a well crystallized mixture of Feldspar & Hornblende. — This latter has a very tough conchoidal fracture 659. 660 highly sonorous, will strike fire with steel & occurs in large masses. —

Excepting on the Eastern base, the direction of the vertical plates of slate

80 verso

(a) From the simplicity of structure of this hill the gradual mineralogical change is beautifully seen. —

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are not disturbed; but at the summit on the N & S sides, the plates lean a little from the hill (or dip to the opposite side) as if the top parts of the hill (a) had expanded & forced the sides out. — It is clear that the uprising of the hill in so level a country, the alteration of the slate, the bulging outwards of the plates of cleavage, are connected together & probably owe their origin (b) to the action of heat from below. — The Tessili is the next highest land to the Mount, although very much lower. its formation is somewhat similar, being composed of Phyllade with the higher portions altered into a more crystalline rock. —

Pampas A few miles W E of the mouth of the St Lucia. I found the remains of a very modern bed; it was situated above the present level of the river & composed of an impure sand, hardened together, & penetrated in every direction by plates & stalactites of a white substance, which is formed of particles of silex, in a friable calcareous base. — I say stalactites because they are long vertical cylinders, which & the plates or veins, withstand weathering & project above the sand. — These have evidently been formed by a posterior infiltration. — I should have scarcely noticed had I not seen on the other side of St Lucia. a long line of cliffs

Pampas] added pencil.

81 verso

(a) Would not the mere weight of the top of hill be sufficient to cause this buldging out. —

(b) 15 miles in a ESE direction from the Mount is the rocky hill, forming the Island of Horses: this is composed (according to Mr Fox) of masses & fine weather born ledges of a Hornblende rock (1475. 1476) 1475. 1476

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which perhaps may have some connection. with the described remnant. — The general character of the country round M: Video is an undulating plain of turf; the rocks are generally covered by a thick bed of earth, the upper foot or two of which is blackened by vegetation; its present position is to be attributed to the action of daily occurrences: as is shown by the occurrence of bone of cattle. some feet beneath the surface. — It is probable it owes its first origin to the degradation of such modern beds. as now exist near St Lucia. —

Shells

At the lower part of the town & under the Mount, there are beds of shells (muscles) such as are thrown now on the beach, but quite above the present level of the river; as they rest on rock. we must suppose the bed of the river has fallen. — Looking at embouchure of the Rio Plata, as compared to other great rivers, there are few islands or banks formed by deposits of mud. Those which do exist have generally a nucleus of rock: At the junction of the Parana, [Uruguay], there is a large tract of alluvial land, which is occasionally overflowed. — But what becomes of the matter carried down in the Plata. Does the Northern current, which is known to prevail in the South, reach this far & thus carry into an extended surface. all the earthy matter washed by the Plata into the sea. —

Shells] added pencil.

82 verso

[sketch]


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