RECORD: Darwin, C. R. Geological diary: Maldonado. (5-6.1833) CUL-DAR33.153-164 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 against the manuscript and editing by John van Wyhe 7.2010. RN1

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.


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33.

The following observations are very scanty: they do little more than point out the existence of different rocks. — The country is uneven, but every part excepting the bare summits of the hills is hidden by a thick covering of soil. In the opportunities afforded by a short ride, I could only note, what rocks occurred; to be certain respecting their relative positions would have required a much longer time. — the most interesting result, is the removing part of the formation (. if not all) of this country from the Primitive to Transition series, or that in which immense beds are found by the mingled fragments of an older set. —The distance I rode was about 70 miles in a north direction to beyond the Arroyo Polanco. & on the road made a deviation through the town of las Minas.

In the immediate neighbourhead of Maldonado. the rock consists either of feldspar. quartz. either by themselves or more generally with green mineral, (uncristallized Hornblende?) which replaces the Mica. 1351 —

The feldspar is almost always red coloured & predominant 1352, the green mineral chlorite (!) is often in very small quantities: — The rock is either compact or has a slightly slaty structure, in which latter case the green mineral forms the planes. — The hills are small & papi-form, arising abruptly from the turf plain: their direction (one to another) is about the same as the cleavage which runs in a NNE & SSW. — (a)

Proceeding to the West. at 5 miles distance we cross the Sierra larga. — this chain rises at Punta de Ballena & stretches it is said, many miles into the country. — The chain is low but very straight &

chlorite (!)] added pencil.

153 verso

(a) Dip when not vertically is either ESE. or WNW. I do not know which preponderates

[Carnation gravilis] plains are said to be remarkably level. Madras Journal of literature & science. October 1836.1

R. N p 72
Limestone passing into schists

Daubuisson V. II p. 153
quartz & feldspar are p. 15
passage of clayslate into Limestone p. 109

1 Benza 1836.

2 Red notebook.

3 Aubuisson de Voisins 1819.

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regular. — it is composed of a main row. & two or 3 lower subsidiary ones. — The hills are extremely remarkable from the perfect straightness of the lines of cleavage; these form white & ribbon-like bands on the sides & summits. — They are best seen where a transverse rally for a short space interrupts their continuity. The lines of cleavage run in a NNE & SSW direction (a) & run may be seen stretching for a great distance exactly parallel to the chain. —

They are either vertical or at very high angle. — The rock is slaty; the divisions being very even; it is composed of ferruginous silex 1350 & specks of mica. — The structure appears mechanical & the rock would perhaps be called a sandstone. I cannot however think so, considering its geological position as a mere stripe between other & certainly crystalline formations; the similarity of cleavage with the others & likewise in some places rocks of an intermediate character. —

To the West of this chain (& at the head of Laguna del Potrero) there is much rock the general rock is such as I have described, as around Maldonado; also some little true gneiss. & decomposing clay slate.

Adjoining & touching to these latter rocks there is a bed of white. semipellucid. crystalline limestone, with conchoidal fracture. 1353 1354 — it contains asbestus, large crystals of Hornblende. & mica in planes; also is penetrated by granite veins: it splits in one direction. viz NNE & SSW. (b) the one common to all these rocks: the limestone lies in a valley between Sierra larga & other hills of the imperfect (quartz. felspar. green mineral) gneiss: it is covered by soil. was discovered & worked by the present owner of a Lime-kiln.

154 verso

(a) Dip WNW Here again occurs the [purple] of Tierra del Fuego; why (putting aside accidental coincidences, which as then are now 2. (if not those with Falkland Islands) instance may be considered banished) the line of cleavage should follow be the same as that of elevation arguing on the common supposition that a range of mountains owe its form to upheaval in that line. —

(b) The marble & adjoining rock dip vertically or at very high angle. — the marble forming a parallel bed in the adjoining slate. The granitic veins did not alter the nature of the limestone at point of contact. —

N B. — direction cleavage same in the flat valleys as on the hills. —

N B. I imagine there is a considerable resemblance between the ferruginous slaty quartz Rock & the formations of Villancia in Brazil.

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Proceeding onwards to Paso de Azucar, the formation continues the same: but the mixture of quartz & feldspar is more pure. — Here large masses consist of nothing else & specimen (1198) 1198 is characteristic of the common variety. —

Sierra de las Animas is the highest mountain in this neighbourhead. — I should think about 1000 feet high. — It lies in an irregular north & south range. which forms the barrier between the level country of the Arrazos "solis chice & grande" (a) & the mamillated plain of Maldonado. — This hill is entirely composed of a pale reddish feldspathic rock porphyry with crystals of feldspar. — it varies in compactness even passing into Hornstone: it contains much iron, sometimes specular. & rarely a crystal of Hornblende & quartz. — It is said, that a man by washing the detritus brought down by the mountain torrents obtained some gold from the hill. — Pan de Azucar a short distance to the SW is made of the above rock together with the mixture of feldspar & quartz in largish crystals, & syenite. —

In the valley between these mountains, there occurs the imperfect gneiss, green slate. & a greenish porphyry with crystals of feldspar. — in another place however amygdaloid with agate. 1199 — Pan de Azucar, like all the hills, is of a globular-conical form. —

Proceeding from this to the town of Las Minas at some distance, we skirted a chain of hills several 3 or 4 miles long. running in a NNW & SSE direction: they are entirely a Breccia 1196. 1197. exceedingly hard & containing fragments of varying sizes of Granite, coloured siliceous rock & green slate. The paste is so full of crystals

155 verso

(a) Perhaps the boundary between the N & S hills & cleavage & E & W of M: Video.

(absence of mica)

(absence of mica)] added pencil.

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of feldspar, that in some places, at first sight you would hardly suspect its true nature. —

Partly perhaps from this cause, the rock weathers into those enormous looking boulders generally so characteristic of granite. — I have no doubt this bed reposes on the continuation of the Sierra de las Animas.

I could not trace any stratification. I shall have occasion to notice again a similar formation. Before entering a broard, mountainous band. South of Las Minas (a), we passed through much imperfect gneiss, & quartz & a some clay-slate.

This irregular mountainous track seems. to run East & West. the rock is entirely a blue siliceous slate; often containing beds of quartz & much iron. — The cleavage (b) was of that extreme tortuous character where almost every possible curvature was present. The forms resembled these forms. seen on dropping a heavy fluid into one of less specific gravity.

A few years since Gold mines were opened in several places, in these mountains from the very small quantities found. they have the works have ceased. —

The excavations were made in places, where beds of quartz contained much iron & some malachite: the stone was full of drossy cavities & quite rotten: — the slate in the neighbourhead was more talcose & of a greener colour. — the gold was obtained by pounding certain portions of the ferrugino-quartz rocks & washing the powder. —

The town of Las Minas is built chiefly on

156 verso

(a) By imperfect gneiss. I mean a mixture of feldspar quartz. with or without the green mineral. — the constituents generally being arranged in planes. — It is a very striking feature in the Geology of the whole of this country the great scarcity & often entire absence of mica. — This is strongly shown in the sand (which is very common) resulting from the decomposition of these rocks. where in vain scales of mica may be sought for. —

(b) with cleavage running about E & W

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a pale & blue slate; there is also in the neighbourhead much quartz. some white. but more generally reddish. — I believe Las Minas takes its name from some gold found in alluvial beds. — ( & lead in Limestone. Mawe)1

dips NNE or to NWN

To the NE of the town there is fine white marble joining (b) & having same cleavage with clay-slate. At Lime Kiln, near to this, I saw a good deal of a bluish limestone, penetrated in every direction by white calcareous veins & containing imbedded fragments as in the transition limestone at Plymouth.

To the East then imperfect gneiss. — & much rock apparently composed of layers 1184. 1185 of reddish crystalline feldspar in slate. — In the midst of this were some considerable hills of pure quartz: —

In the neighbourhead of Minas (c). there would appear to be no great uniformity in the cleavage or ranges of hills. — Again further we come to much beautiful semipellucid marble; lying as we have so repeatedly seen it in clay-slate. — The general rock where not specified to the contrary being imperfect gneiss. — All what I have now described lies within four leagues, to about E by N of las Minas. —

To the South & East of the sources of the [Arrogo] [Marmaraja]. there a very extensive Breccia formation somewhat similar to that described at near Sierra las Animas. — It extends for several miles, in the form of rugged hills. — The rock varies from one so compact, as to resemble jasper, to the coarsest breccia; the base is of a bright red. (a) — & many of the varieties strikingly resemble

1 Mawe 1825.

dips NNE or to NWN] added pencil in margin.

157 verso

(a) especially when a sandstone. — I omitted to state. that I could nowhere trace any certain stratification. —

(b) which dip at high ∠ to the NNE or more northerly

(c) to the East of Minas. no S ranges of hills & cleavage seemed to predominate & to the North E & W range — Amongst the limestone & slates; there was a blackish slaty limestone dipping at high angle to the SE. —

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the old red sandstone of England. — In returning across this range I found it lying on each side of some central granite. To the South it is separated from the crystalline rocks, by it a band of greenstone has (such as 1186) & amygdaloid (such as (1192). — To the North there is a tract of country composed of coarse true Granite some quartz & much syenite, forming disconnected conical hills. —

These rocks separate the sources of the Marmaraja from the Arrogo Tapes. — In the basin of the latter there is a formation 3 or 4 miles in length which is very remarkable in the varied mineralogical S nature of its rock. — Regrade within. The commoner are greenstone porphyry & amygdaloid (specimens 1186 ... 1195). 1186 ... 1195 1194 —

Tapes

But the varieties are endless; & what is the remarkable the whole would occur in the space of a few yards. —

Perhaps specimen (1194) was most abundant: the greenstone or Basalt (1186) formed small columns, but not perpendicular to the surface. — This formation lay at the base of a range of hills of the imperfect gneiss. — Intermediate in position, there was a singular variety; a red porphyry. with dousy cavities 1195 lined with crystallized quartz; & these cavities were irregularly arranged in planes, so as to give a certain air of cleavage to the external surface. — these planes had a direction of N & S & dipped to the East at an angle (estimation) of 45°. At the distance of a few yards, ascending the slope, the first rock which appear is an imperfect gneiss, formed only of feldspar & quartz but arranged in imperfect planes. The cleavage here had a direction (within a point; for it was not easy to be certain) of N & S: Ascending the hill

Regrade within.] added pencil.

Tapes] added pencil.

158 verso [blank]

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this gradually died away, & there remained a mixture confused mixture (such as 1198). of crystals of red feldspar & quartz. (a) — Now in their chemical nature. I should think this imperfect granite. & the porphyry with drossy cavities would not be widely different. — It is certain all these Trap rocks lie fairly within a granitic basin. but what are we to consider them? as a contemporaneous formation. such as the above appearance of curious transition would point to; or, as the nature of the rocks would render more probable, as a posterior one. — I must lean to the former opinion. no —

On the other hand, the occurrence of a band of similar trap rock. separating the Breccia from the Granite has been noticed. — Being unable to find any junctions. I know not their true relation; one would be inclined to imagine, that the Breccia & Trap rocks were intimately connected. — With respect to the age of the Breccia. I am equally ignorant. — no external features in the country point out any great difference between it & that of the imperfect granites &c; from their position immediately upon & within the latter formation; from their mineralogical characters, I think, the same conclusion would be drawn. If so the whole series of the country must be placed in the Transition class (b); where we might expect great beds of Breccia. & where perhaps we might more readily believe in a transition between the (volcanic looking) Trap rocks & the Granites. —

Proceeding North from the Tapes to the Arrago Barriga

no] added pencil in margin.

159 verso

(a) I omitted to state that the distance from the imperfect gneiss to the ordinary trap. is not more than 50 yards; the intervals being covered with turf. — The exact situation is on the banks of a stream. which forms the source of the Tapes.

(b) As a weak argument, in favor of this view the Limestone at the kiln near las Minas, which contained fragments again united, may be brought forward. — The workmen said they had never observed shells in this rock. —

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nigra there is imperfect gneiss & some fine granite (a), with bright red feldspar. — there was likewise some limestone. After crossing the Barriga nigra we again continued to find beautiful granite with large crystals. — & then at the house of Don Juan Fuertes entered upon the Limestone district. — Here for some miles to beyond the Polana the rock is either quartz or granite & much limestone. — The Limestone is the same as already described being either white or blackish with white vein & with the other usual characters. —

It contains veins of quartz. & Rhomboidal gypsum. & likewise asbestus. Mr Mawe in his "travels in Brazil"1 talks of two sorts of Limestone as occurring here; one of a dull blue colour overlaying the bases of the mountains & the other as forming perpendicular laminae. — I could perceive no difference in the geological position of the different limestones which I saw, althoug varying much in colour. — The workmen at the Lime Kiln assured me the different sorts passed into each other. —

The same men said. that a few miles North of the R. Polanco. there was extensive tract with no other rock excepting the Limestone & that beyond this again came the Granites. (c) —

The Limestone alternates frequently with the Granite; in returning in a SSE direction. I crossed, 1st section could not have been more than 3/4 of a mile. —

The only part of the country which I have yet

1 Mawe 1825.

160 verso

(a) I believe the beds were nearly vertical

(b) From the source of the Marmaraja to the Barriga nigra. a decided N & S. mountainous range. overlooking the flat Eastern country. is said to continue up to Portugal (Brazil), the course of waters point out same thing. —

(c) The workmen stated that there was much gypsum in this part (3 or 4 days ride northwards) & Mr Mawe states he found some not far from the Barriga forming a little plain. [Nov.] 1833 — perhaps probably relies as the Breccia is, of the great modern formation of the R. Negro..

The Trappean formations of this district are interesting from their extreme rarity: Humboldt after describing in description of the formation of Pyrox. Amyg: & Phonolite between Ontey & Cerro de Flores says "I have often directed the attention of Geognosists to the absence of Pyroxene Porphy .. Trachyte — Basalt Lavas in the whole of America Eastward of the Cordilleras. Vol VI P 646. —

1 Humboldt 1819-1829, vol. 6, p. 646.

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to describe is a section from Las Minas to Laguna del Potrero near Maldonado. — This of course lies to the East of the one from Pan de Azucar. — The rocks were generally the same. being imperfect gneiss & slaty quartz & quartz. We had however but very little of the siliceous slate, which as already described. a little to the westward forms a mountainous band of several miles in breadth. — in neither sections could I understand the structure of the hills; in each one hill or short range the cleavage was parallel to its direction; but these so varied, that it the tract appeared a wilderness of low mountains without order. (a) v

It would appear (b) like the regular NNE & SSW ranges of the southward & more level country, crossing another & distinct range of slate. —

Alluvium
I have alluded to co thick alluvial covering as throwing obstacles on geological research. — This is generally very earthy & shows but no signs of regular disposition: yet in one deep section. the upper parts were much more argillaceous & they rested on a regular bed of white sand. — I have also observed in numer very many places the occurrence of numerous white calcareous concretions 1355: & other arenaceous ones with much iron. — These are best seen where the alluvium has been washed away. they then lie in heaps. — I have seen something similar at M. Video; likewise V P 68: These facts convince me. that the great bed of earth was formed beneath the water: I never could see any organic remains

161 verso

(a) I should estimate the height of these hills to be from one or two to six hundred feet above the level of the plain. & this latter cannot be much above the sea. —

(b) At Punta L Este there is some true gneiss. the constituent parts unusually well developed. —

At Gunthi Isla. — more generally imperfect & siliceous varieties: differing much in colour. — Clea Also some true gneiss cleavage rather more Easterly than what is common vary about NE by N & SW. by S. — There is a pudding stone forming at this present day near beach, composed of pebbles of the coarse sea sand. cemented by a calcareo-ferruginous base. — it is literally hard. — (I should imagine it was formed to under cover of sand dunes. & water filtering through them becoming impregnated with mineral contents, & the then dunes have subsequently been removed ?) — I was formerly inclined to believe that modern Breccia did not occur excepting where volcanic or trap rock was near. this is a strong exception. —

(a) In the Eastern part of the blue slate forms a band about 10 miles broard: tortuous cleavage running about E & W. — to the westward of this there is a little slate & cleavage (about gold mine &c) more NNE & SSW from these to Pan de Azucas. — the confusion seen in hills in the more Western section arises perhaps (& tortuous cleavage for cle the two chains I believe to have been formed together. —) far from the crossing or junction of the Western & NNE chain of hills. (where the slate & granite join). — the course of water partly point out same fact. —

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Laguna
Some circumstances concerning a fresh water lake about a league from the town interested me. — It is only separated from the sea, by the a bar of about 30 yards across. it is said that this annually breaks down for the passage of the fresh water. — After the ? of rain, which fell in May, the lake was very full. — the level of its water was between 8 & 9 feet above mean height of the sea. — the water oozing through the bar undermined it & gradually the a great part fell in, Much of the water of the lake was thus suddenly emptied, & as far as I could judge its mean height corresponded with the sea. — that is high water was above its level & low, beneath:

During a gale of wind the sea broke over the low & newly repaired bar; & thus a current of salt-water setting in made this end of the lake brackish. —

From the same cause there were many marine shells, mingled with immense numbers of large Ampullaria 1339 the animals of the latter dying & dead from the effects of the salt. — By the sinking of the lake, a large surface before covered with water was exposed; in the mud the number of Ampullariae Limniae &c. Fish. Craw fish was exceedingly great. —

At the head where a stream entered (likewise before covered) about 3 miles inland, a section was is exposed of muddy clay. bed. containing numerous Myae (?) buried vertically as when alive, likewise another shell & a small Turbo 1312; all of which appear to me to be the same as now exist on those

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Laguna
mud banks at Bahia Blanca. which are covered at high water: — there were are likewise fragments of Mytilus which yet retained their colour. — All these shells were are in a very soft. & decomposing state. This bed was is covered by another of black mud about 4 feet thick formed evidently by the lake or stream. — The clay bed with shells is one or two feet above the level of the lake. — & the lake, as we have seen is below high water mark. I think therefore the tide if it were to enter by a large passage. would flow above this level. —

At the bottom of the lake there must be a bed containing great numbers of fresh water shells (& near towards the sea end of it a curious mixture with marine ones) & Beneath this fresh water deposit. the bed already mentioned unquestionably extends; (perhaps the shells in it would not be such as live on mud banks, but they of course marine). —

This fresh water lake is deep; so how certainly if by change of currents produced by wearing away of headlands &c &c the sea surf should fail to keep up the narrow barrier of sand would the sea take possession of this inlet; marine deposits would be formed, perhaps again to be covered by a fresh water lake. —

I have been particular in noting these circumstances because perhaps an arm of the sea & banking up the fresh water far above its proper level might explain some of the alternations of the two classes of fresh water & marine beds

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Laguna
We have seen something similar near Rio de Janeiro, where a band of about 5 or 6 miles in width of lakes & marshes skirts the coast. V P 40. — Of course some more efficient cause would be necessary to explain some of the very extended formations where marine & fresh water deposits alternate. — They can however occur as we have seen, without any sort of change more than is going on at the present day. —

July
On looking at the most southern part of the map of Brazil, there will be seen two great lakes connected with each other & the sea. — From what I can understand collect (& it is probable from their structure) they contain fresh water (B). excepting at the connecting channel or Rio grande. — It is also The tide however extends to within the lake. — It is also said. that they the separating the stripe of land. which separates them from the ocean is nothing but sand-dunes. This likewise I should have expected from their structure, especially the long peninsula. in which is Laguna Manguera. — Now these lakes. viz Laguna [Mensin] & de los Patos (a) extend for nearly four degrees of Latitude. & including the bar at a width between 40 & 60 miles. — I should imagine on the principle above described, there would at least be one alternative a freshwater stratum above a marine, if the country was elevated & here there would be two great basins. viz that of Laguna [Mensin] & de los Patos, of alternating strata without the necessity of requiring frequent up-heavals & depressions. — From what I have observed near Rio, doubtless in such lakes there would be an abundance of shells. —

164 verso

(a) a distance about as great as between the Isle of Wight & Paris Basins. —

There is a bar at the mouth of the Rio Grande. —

(b) I hear the water is occasionally salt 50 miles from the mouth —

It is very striking how exactly what I imagine must take place here agrees with the required date to explain Paris basin according to Lyell. 3d Vol. —1

Absence of Mica

1 Lyell 1830-3, vol. 3.

Absence of Mica] added pencil.


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