RECORD: Darwin, C. R. Geological diary: Lacuy Peninsula. (1.1835) CUL-DAR35.331-340 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online,

REVISION HISTORY: Transcribed by Kees Rookmaaker, corrections and editing by John van Wyhe 5.2011. 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. See a map of Chiloe from Narrative 2. Darwin's time in Chiloé is described in the Beagle diary pp. 280-6.

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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.


Jany: 19th. — Lacuy. Peninsula: — 1

The beds on the northern part are essentially the same as at P. Arena: The beds are rather more regular: strata of sandstone, which occasionally is yellow grained (as mentioned) or common & brownish (2514), which & is perhaps is the more common, alternate with numerous layers of a greenish slaty indurable clay. — This sandstone often passes into or contains masses of the breciola with pumice-like fragments. — It com is thickly strewn with [illeg] concretions of a harder, more crystalline structure:

I saw this quite sphaerical one (like 2513) but about two feet in diameter, it was entirely filled with loose ferruginous sand. — The crust alone being the more compact nature. — Surrounding a large concretion there would be a necklace of small ones, united together thus making a mamillated appearance border. These would unite into an elongated arm. — Viz. [sketch]

The sand washing out of the inside of these concretions form curious little pits in the horizontal section near the beach. — The sandstone is divided by numerous siliceous & even a perfect agate plates, which fill up numerous rectilinear fissures. These intersect each other generally at a high angle: Even where agate siliceous plates

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are not present, there has been some subsequent infiltration for the borders are in all cases hardened. — These fissures must have been formed before the setting of the yellow sandstone. — For a horizontal section (Diagram β) shows that the each figure bounded by the lines of fissures contains a concretionary nucleus. — This structure is very prettily shown by the sinuous lines of arrangement of coarser particles & shades of color. — It will be seen there always is a symmetry between the arrangements of sandstone of the lines of fissure: sometimes these lines determine the centre of the sp curves: but generally they bound separate them: at others, they form bisect the tops of a wave: — undulating lines. — The drawing is an accurate representation of a piece 3-4 feet x 1.5 or 2. —

In the sandstone there were large pieces of detached of carbonaceous matter (or blackened wood) & others of a purely siliceous nature. (2318: 19) I only found one in situ, but several were very large pieces (20 inches)? — on the beach. — The connection of the agate plate & this latter process fact is manifest:

I can hardly believe that the siliceous matter has percolated through the sandstone & served on certain pieces to deposit itself. — If we allow this, the

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[calculation] 45 [-] 17 [=] 28

Jany. 30th. [1835] — Went up the Harbor to the Westward, in a line of the strike of strata from Tenuy Headland, viz S 17 W. We find the same alternations of sandstone & slaty clays, which dip exactly to W 17 N ∠ 10° or 12°. — But further on the perfection of the observation is spoiled by finding inverted saucer shaped stratification (reminding me of Bahia on the coast of Brazil) where the more prominent dip is W 28 S. — & some even dipping S. = This if anything casts a doubt on the observations. Yet the coincidence in line is very striking. —

There here occurred another variety of the breciola so often mentioned (2534)


Lacuy. — Peninsula (3

Agate plates must be contemporaneous; we have showed the fissure to be so with the sides of which they seem almost to blend to be so. —

In the slaty clay beds, numerous fine fissures in every direction are coated with delicate fibrous gypsum, something in the manner described at S.Cruz. I saw only one & an angular pebble in the sandstone. —

Entering in the ship the inclined strata at Point Huapacho & Tenuy are very observable; a fact so rare in the tertiary formation of S. America as to be worthy of particular examination. — At P. the headland Huapacho we have nearly horizontal strata of a very white sandstone, or rather it is a white friable aluminous substance (perhaps containing magnesium such as (2516). — A little to the West of this (V. Chapt.) we have a mass of strata dipping at 16° to W by N 1/2 N (P. Tenuy). There are three little headlands points which give 3 excellent sections. — These inclined strata form a little ridge on the side of the headland, a small valley appearing to separate them from the horizontal strata on the East. — I will first describe the mineralogical nature. —

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diagram (γ) on the East side a mass (100 ft?) high of numerous regular alternating beds of brown sandstone (2514) & green slaty clay dip to W 17° N; ∠16°. They preserve during their whole dip same thickness, — on the West, there is a nearly amorphous mass of a breciola, the specimen (2515) is intermediate line; much of which is of the nature so frequently mentioned with bits of so called pumice: the upper part passes into a remarkably white friable aluminous substance (2517 :16) somewhat resembling the great upper bed of the Patagonian formation. — There are few lines of sphaerical concretions & others of stratification parallel to the [illeg] dip of other beds. — This bed (as shown) caps conformably the sandstone, & then gradually becoming soft & red from ferruginous matter (specimen 2515 is intermediate in its character of the breciola & upper strata on East side), is divided into regular strata. The whole of the Eastern strata in upper parts are so colored. —

The junction of the breciola & strata of greenish slaty clay is very remarkable. — In the lower part the strata are continued by an irregular line

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to (A) variously shaped, but elongated, rounded pieces: I know not whether owing to mechanical or concretionary origin. — at (B), a stratum terminates exactly as represented. — In another section (δ) about 400 yds to the South or strike of strata, — The same general facts are present: The termination of the greenish clay is here by a mass of fragments, penetrated by the breciola & this overlies the pure strata. — Above the white friable aluminous matter in this section we have a repetition of the sandstone & clay beds but their dip is not quite so great or regular. — On the East — I presume these 2 junctions (the appearance of which is rendered singularly deceptive in the first instance the veins fissures which resemble faults & are owing to a current during the deposition of these beds bearing the brecciola having partly removed the sandstones &c when soft. — The point (b in γ) is very problematical. — We shall afterwards show that at some no great distance a mass of pebbles replaces the finer sediment & proves the existence of such former currents. —

These white beds occur in same in the sections occur in the same exact line, which is also the strike of beds:

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P. Huechuenuy — hard rocks. —



In a similar manner on the East side in each case the strata contain much ferruginous matter. —

I think this most projecting headland has very certainly been formed by a line of elevation running N 17° W, tilting on the West the strata, but leaving those on the East horizontal. — To the West the shoal of Huapacho is probably the remains of a similar crest:

Two & half miles to West, there is a small point, (East of Huechuenuy) where from the ship I saw similar strata capped, by white beds dipping like roof of house at a considerable angle; this forming an other parallel anticlinal line. —

To return to the East of horizontal strata of P. Huapacho, we again meet the West dip it here forms a narrow point, with long reef, formed of similar sandstone & clay beds, with the white beds. —

The line of low hills is very narrow. — At the highest part the dip is 40°. — from this it gradually lessens till at base it is only about 12°-15°. [sketch]

Form of hills: This occurs at the East & West base. On the East base side, the I found the strata dipping to the East at small angle; this was not directly at base of point, for the strata continued to dip West till concealed by the water; but a few hundred yards to the South, & to the

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East. — Therefore the anticlinal line here ran along the Bay. — V (E). —

The strata where the East dip was occurred, chiefly consisted of gravel, where the lines of pebbles showed the dip. —

Again at P Huapi Lacuy, the strata have a West dip. — We thus see in an E & W Band of about 4 miles broard, the strata furrowed by several N & S lines of elevation. — I think the most probable explanation of this lies will be found in the supposed several parallel, concealed N & S broard dykes: — It must be remembered at P. Tres Montes. volcanic streams lava divided by sedimentary beds, as which in many respects are exceedingly similar to those near town of S. Carlos, are intersected by very many & nearly parallel dykes. — The strata dip from these dykes. — Now here we have no lava, but sedimentary beds, originating in volcanic phenomena; at no great distance we have the true lava. Is it not probable that beneath the ground, some grand dykes here have broken & traversed the strata, in parallel lines, which have run N & S by E 1/2 E, as these NE by N & SW by S (?). —

N, 17 E. — N 34 E. —

When a volcano is partly choked up, after having been long in action

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X The extreme regularity & uniform thickness of strata the anticlinal dip at point E of Huechuenuy. —



The neighbouring country would probably be traversed by dykes.

This is what has occurred at Tres Montes & is supposed to have done so here. — This fact of a bands of trouble country amongst the immense extent of horizontal strata, immediately calls to mind the celebrated line of upheaval at Isd of White. —

I feel no doubt but that this is the true explanation: but at the same time I am bound to state: that in spots between these lines, there was a sort of saucer-shaped stratification, which is quite independent of upheaval. — This cannot fail to cast a shade of doubt on the subject: to which may be added the singular bands of mineralogical change parallel to the strike (in section γ & δ) & the manner of junction of the two substances on the Rt: hand. —

At first sight, this line of pebbles would seem clearly to prove that they had originally been in an horizontal plane. — But the near very beach on the spot is composed shingles, at inclined at ∠ of 20°. —

On the Beaches in this district there are but few boulders. — In the chart, a low tract of land with a lake separates the high land of almost form the peninsula, on which above described. — Here we meet numerous some magnificent boulders

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Road to Huechuenuy few boulders all places



about as large as the second size of Chacao. — & composed of syenite. — It is the variety pointed out to me by Mr Douglass, as forming base of Cordilleras of near Cal [Relacavi]. — I feel no doubt these have come from that site (distance), however there may be some concerning the granite ones near Castro &c &c.

The situation of these boulders is strai á land strait which certainly must once have been a valley channel of the sea, recalls to minds, the very similar facts at T.del Fuego. — One of the largest masses is on North side & about 150 ft elevated: All the boulders clearly have been washed out of the neighbouring cliffs — for several may even now be seen imbedded in the & them. In the chart a dotted lines shows the separation of gravel & a grand mass imbedded in sandy mass — from the northern strata such as described:

the land on each side of this line is of same height: & although I did not trace actual junction I have no doubt it is most probable (a) there is passage from the finer sedimentary beds to this mass (of so called) alluvium, which such as has been so frequently described on the E. coast. — We must suppose

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(a) On the finer sedimentary mass might have been removed & replaced by the gravel: it does not make much difference, for with respect to my arguments. — For the gravel replaced the other strata so as to make all near equal in height.

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this band of gravel, traces the line of a current in the ocean which deposited all the above strata. — It is remarkable the coincidence of this line with the subsequent passage, excavated during upheaval & now forming dry valley. — It shows how gradually the face of the land has assumed its present form — an ancient antient current by forming a line of deeper soundings has first formed an Island, & then a valley. —

It is in this gravel I suppose all the boulders to have been imbedded, & that they have remained within the strata of gravel. — I cannot prove, that the some of boulders have not been subsequently carried here. — But although lying on the beach of English Harbor, it is clear from the one elevated one, within the land strait, that they came at level the latest during the time, when this was a deep channel. — & I to repeat we are sure, that some came were present brought with the gravel. —

NB. The sudden alternations of very fine sedimentary beds & coarse alluvium without organic remains seems to characterize deposits in rapid water ways, in shoal water — hence Sts of Magellan: caused by eddy. —

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