RECORD: Darwin, C. R. Geological diary: Geology of Monte Video. [7-10.1832] CUL-DAR34.3-6 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online,

REVISION HISTORY: Transcribed by Kees Rookmaaker, corrected and edited by John van Wyhe, corrections by Gordon Chancellor 11.2011. 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. See the Beagle diary pp. 198-213, 246-247.

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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.



Geology of Monte Video

The rock consists of two sorts Mica slate & Phyllade (Daubusson).1 — Mica slate may be known at a distance on the coast by decomposing & forming sand hillocks. —

650 651 Rat Island

Mica slate is generally coarse & quartzose. — Containing layered veins of quartz & much intabgled by them in places. — This rock passes into Hyalomictite (Daubuisson) 661. 662 mica silvery & quartz granular. — it presents irregular projecting masses. — The to stratification is the same as the next rock. —

Phyllade. this is has seen on the Mount. — Here it frequently alternates with mica slate: the beds vary in thickness sometimes only a few yards: & again forming the whole Mount. —

648 649 Rat Isle

At Rat Island small pieces are enveloped in Mica slate & most curiously penetrated by incurred veins of quartz. this is rent off from the mical slate but here the planes

1 Aubuisson de Voisins 1819.

3 verso

Octob. 28th. — (East?) or further section of the town: — present alternations of Mica slate & phyllade. — varying NSE or NNE. dipping to SSW ∠ of 2°. — Intersected by irregular beds, dykes looking of quartzose rock, containing large crystals also of feldspar &c.

plains bet almost composed of it. —

[in right margin] The same principle occurs at Rat Island

In places the Mica slate, contains numerous bands, parallel to the cleavage of more quartzose rock: & in this case was curious to mark the transition of Mica slate into Phyllade — the intervals have the quartzose rock collecting into bans & the micaceous into phyllade. — in those places when crossed by masses of siliceous rock, the quartzose bands were constructions joined to it ∴ their masses are contemporaneous & only differ in not having a cleavage. —The fact of gradual transition where the bands exist, show that the separation of all the beds is only a more perfect & similar process/ — & this process chemical; not depositing. —

as this takes place in line exactly parallel to cleavage, may not cleavage be a sort of crystallization. —

If the rock were deposited horizontally; there would be some difficulty in understanding how a small vertical piece of different rock. should so closely connected in its nature, as to be [united] at siliceous bands: [sketch]



of cleavage & fracture being the same as mica slate shows it to be a contemporaneous formation. 652. 653 654

The slate is generally fissile. soft & greenish tinge. — sometimes the mica tending to pass into Chlorite, as it does in others into Hornblende. — The rock becoming Hornblendic slate. — 655

The cleavage is well marked generally being running E & W (true). — this place is the its dip is vertical. This applies also to the stratification as shown by the junction of the mica slate to phyllade.

Besides these similar planes of cleavafe & strata, there may be seen (especially on the coast) both smooth parallel plans of fracture crossing those of cleavage & nearly diping crossing those [illeg] of cleavage nearly at St [X]. dipping generally to E at ∠ 20 nearly to one of the two points of direction. This was generally towards E & at an angle from 10° ... 20°. — Thus forming steps on the edge of plates of cleavage.

This also occurs in a lesser degree in the mica slate. — All these observations made on the level base at the Mount. —

4 verso

(a) Measure it Barometrically



Monte Video

The Mount is 450 feet above the sea (a) & the higest land in the country. — Its longest axis is in direction of strata viz E & W. —

Ascending to the top the slate gradually becomes more compact harder sonorous. slaty conchoidal fracture.

656. 657
658. 680
681. 682

at last it is converted into slaty greenstone which in some places is a well crystalline mixture of Feldspar & Hornblende. — 659. 660

Excepting on the Eastern base the direction of the vertical plates of slate are not disturbed, but at the summit on the North & South sides the plates lean a little from the hill (or dip to the opposite quarter side) as if the the summit had been expanded & forced each side out. — // m

If only one side had been seen it would have been said that the force which raised those beds. had here a little failed. — but this cannot explain the leaning in opposite directions on opposite sides of hill: —

It is clear. — that the upraising this

5 verso

The following is the direction of the cleavage of the slate. whilst walking round the Mount. —

The bearings are taken from the Lighthouse. — both these & cleavage magnetic

Bearings Cleavage    
E by S E & W Phyllade near the coast
E by N ESE & WNW Alternations of M Slate & Phyllade [do]
NE by E   Hornblendic slate [do]
NE NE by E (?) Phyllade [do]
NNE do do [do]
NNE E by N altered slate near summit of hill
NE by E do do [do]
E 1/2 S E by N 1/2 N slate phyllade [do]
WNW E ny N altered slate [do]
W by S E by N do [do]
SW do do [do]
SSE ENE do [do]

X is this owing to some salt from sea —

water remaining in the earth? —



hill in so level a country: the alteration of the slate: the bulging outwards of the its planes on the summit are connected together. & I should think might be explained by the action of heat. — It may here be observed how strange it is, that in a country which has suffered so remarkably little from the convulsion of nature, that the stratification should be vertical. — How is it possible that horizontal plates, deposited beneath water should thus be elevated through a space of 90°. — & yet the country be one of the most unbroken on the face of the globe. — The Mica slate in Rat island is crossed by small dykes of greenstone, but so much decomposed it is hard to say what it is. — (N B. Rat on island is in same direction of longest axis of the Mount). —

It is clear the Rio Plata has had much larger limits than at present. — large beds of muscle shells occur beneath the mould in several places in city & on the Mount. — Again in dry summer the little brooks. in the plain are brackish. X

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