RECORD: Darwin, C. R. Geological diary: Chiloé. (7.1834) CUL-DAR35.206-217 Transcribed by Clare Ring and Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)

REVISION HISTORY: DAR206-211 transcribed by Clare Ring, checked and corrected against the manuscript by Gordon Chancellor, transcription adapted by Kees Rookmaaker, DAR212-217 transcribed by Rookmaaker, corrections and editing by John van Wyhe. 4.2011. RN1

NOTE: Darwin was in Chiloe from 28 June to 12 July 1834 which is covered in the Beagle diary (ed. Keynes), pp. 245-248. Chiloe is mentioned in South America, pp. 27-31. See also Darwin's 'Chiloe Janr. 1835' [Beagle field notes]. CUL-DAR35.328,328a-328j

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.


206

35

1834 July. — Chiloe 1.

In the neighbourhead of S. Carlos all the rocks are of volcanic origin. – Nearly half of what is seen is in the state of a fragmentary mass, which covers up the bed of Basalt igneous rocks. — The nature of these vary’s much; the commonest variety is a compact heavy black basalt: this generally affects an angular cleavage: the more superior layers are vesicular 2086 2087 2088 (occasionally with amygdaloid lumps of carb: of lime, the external surface of which is very sometime a little radiated): it is here oftentimes in curvilinear plates: where in one place the inferior mass of was of rather a different variety, these curved plates most artificially resemble altered rocks bent by an inferior protruded mass. —

The next most abundant rock is Pitchstone; it occurs in very great quantities (a) forming whole cliffs and beds: it but seldom occurs so compact as in one of the specimens: The varieties in lava seem to pass into each other without any sort of determinate order: The three varieties (2092. 93. 85) occur in close contact with each other. 2092 2093 2085 — a red porphyry (2089) 2089 joined on to a black basalt with large crystals of feldspar .— There are other pale coloured less crystalline varieties. 2090

The manner in which the above rocks passed into a more or less darkly colored clay or wacke is remarkable. 2091 —

The transition is by no means generally in a horizontal plane; a pap of basalt (as seen in a vertical section at the coast) will

206 verso

(a) I should think finding so much pitchstone in rocks which certainly have been poured out in melted sheets & which, without much doubt are of very modern formation (the Craters of which still existing?) is interesting in as much as according to Jameson, it belongs to secondary formations1

Chiloe concretion
Daubuisson p 318 Vol 12

Points of land in Sumatra are called Pula which tradition says were formerly islands

P 31 Marsden Sumatra3

Mem case at Galapagos Isld of [sketch] strata affected thus

Lyell Vol I (1st Edit) p 3464
talks of lava being like clay

I feel no doubt about brecciated white stone at Chiloe. after seeing the process of change in pumice of Ascension RN5

Chiloe stratification
RN p. 87

1 Robert Jameson (1774-1854), Scottish naturalist and mineralogist who taught Darwin at Edinburgh University. See the Buenos Ayres notebook p. 14a, note 1.

2 Aubuisson de Voisins 1819.

3 Marsden 1811. The edition Darwin cited here is unclear. See South America p. 135 and the Edinburgh notebook and Notebook E.

4 Lyell 1830-3, vol. 1, p. 346.

5 References to 'RN' are to the Red notebook.

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1834 July Chiloe (2)

rise abruptly to the middle of the wacke. — The change is very sudden, the latter rocks containing a few nodules of the crystalline basalt rock whilst the basalt close to the contact will only be a very little softer. — a concretionary structure which may exist in the harder, will be less distinctly seen in the softer.

The Wacke generally is porphyritic with earthy minerals. — (x) x. From the abruptness of disposition of the above passages, after the degradation of the wacke masses of basalt stand out isolated on the beach; the geological connection of these with the surrounding beds at first quite baffled me. —

x> With respect to the origin of this substance I can by no means believe it to be the result of weathering of the volcanic rocks.—

Leaving the porphyritic structure I cannot think that water could penetrate so far through clay beds or that the change would be so sudden and of such forms:

absence of crystalline structure appears the only difference between the two substances: there is great difficulty in believing these clays were once molted rocks & that owing to their mineralogical nature & rapidity of cooling they did not assume a crystalline form: the latter cause by itself clearly could not produce the difference, otherwise the change would be horizontal parallel to the cooling surface. — The relative position of the wacke of to the hard rock is I believe exactly the same as any one of the varieties to an some one other. —

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208

1834 July. Chiloe 3

On the line of coast South of the Anchorage there are other varieties, of which the above annexed specimens are characteristic. 2096 ... 2099 — The highest hills in this neighbourhead are all (n) formed of such rocks, a grey fine grained laminated variety is most abundant 2100. these hills are perhaps 500 to 700 ft high. —

I saw the lava in one place separe traversed by a horizontal bed of pebbles; hence there must have been more than one stream. — The coast line is formed of this rock, therefore I could have but few opportunities of seeing what underlies it.

In one place I found numerous thin horizontal layers of muddy sand, yellow clay, fine breccia & laminated soft black clay-slate; these beds passed into a coarse fragmentary mass, which perhaps overlays another inferior stream of lava. — Exactly of the same nature, were but with rather more of clay slate were some small beds which were superior to the mass of lava; I shall give reasons for believing these beds are of the same with the formation of Punta de Arezia Peninsula de Lacuy (NW of the town)

On the road to Castro, on the highest part there was a thick bed of yellow clay, which partially resulted from the degradation of the wacke & the others of hardened conglomerate. the pebbles being of same nature as will be presently described presently, (& owing their origin to the Andes?)

Generally the stratified beds, and the lava was covered by a thin mass of pebbles & which must have been deposited, when the strata were beneath the sea. —

208 verso

Jun: 1835 (a) As far as San Antonio 7 or 8 miles on the road to Castro occasional patches of such rock may be seen; one of the last variety is a greenish-slate colored compact basin with elongated crystals of glassy feldspar 2522;

After this we have some true greenstone, which may either belong to the volcanic or primitive series of the Island. —

209

1834 July Chiloe 4

I have already alluded to the fragmentary masses (a), they consist entirely of the above varieties, the fragments are generally large and quite angular. — The mass, in many cases, is I think, united by the Lava itself; it generally coats it, but sometimes appears confusedly mingled with the formerly fluid rock. I could not trace any line, where it was probable that an aqueous cement took the place of the igneous. —

A moving stream of lava, bearing on its surface heaps of fragments, would entangle some in the liquid mass, whilst others, would might be cemented by mud &c long before the lava heat had been conducted away. This hard coarse breccia passes in another where the basis is softer, & appears to be volcanic mud or ashes, containing smaller fragments of the same rocks as the other kind. This forms high cliffs.—

In the few places where I had the opportunity of observing the lava & adjoining beds, they were pretty nearly stratified horizontally: also, here & there level pieces fields of the solid lava might been be seen: but making allowances for the entire removal of the wacke & the its own degradation the original stream must have been extremely irregular. On the Peninsula more is present, whilst 2 or 3 miles to the South, it forms high hills, perhaps this may be owing to difference in elevating force. — The contrast between the lava up the R.S. Cruz & here is very striking; in the former in long distances, there was but little mineralogical change

209 verso

(a) In one part of the hardest fragmentary mass these several tortuous vertical veins, the thickness of which varys from a few 1/10 to a inch & 1/2 2095

the substance is membranaceous and dark coloured. is it micaceous or carbonaceous ?? — 

210

1834 July Chiloe 5

& the beds were most regular, even & simple, here all is in comparative confusion. — Perhaps part of this difference may be attributed to the supposition that the Lava here flowed over an uneven bottom, whilst in the S. Cruz it must have been quite smooth. —

The peninsula of Lacuy, wherever I have examined it (in the neighbourhood of P. Arena) consists (a) of various soft rocks slopes, with harder concretionary masses, all of these are more or less fusible & I believe either owe their origin to the degradation of volcanic rocks or more probably to dust and matter ejected by volcanoes.

The land rises abruptly and attains an height of about 200 ft. — Following a creek which lies to the West of P. Arena, the general stone is a more or less compact earthy yellowish substance earthy sandstone; which under the Blowpipe is fusible. 2114 2115 — very commonly this forms the basis of a breccia where small angular fragments of a white substance are very abundant 2116: 2117. At some distance from this place I found a coarser breccia, where fragments of volcanic rocks & quartz was included; & frequently sometimes it contained small bits of brown pulverulent wood, which emitted a flame. — The nature of this soft white stone I am (b) ignorant of: I at first thought it a kind of pumice but it is scarcely fusible.

In this white breccia there are many large, rounded concretionary mass of a calcareous 3 & 4 feet thick

210 verso

(a) The concretions are of various natures: some are most singularly sphaerical & exactly resemble a shot imbedded in a soft stone; these almost always are filled with loose sand, instead of being compact in its whole diameter. 2513 one was about 2 ft in diam: —

Jan: 1835

I saw one large such concretion (3-4 feet long), which consisted merely of a shell of stone with included clay. — This occurred in the Brecciola of white Pumice (?). —

The general stone is a coarse, soft, compact, earthy dirty pale yellow, fusible sandstone, or rather indurated dust from volcanoes. —

(b) Towards Catimara I found another variety (2534) 2534

(a) I also noticed, that a large sphaerical concretions, would sometimes have its equator surrounded by a necklace of small ones united together; & these apparently of their union. would form an arm or cylinder, branching off as represented. — [sketch]

211

1834 July Chiloe (6)

sandstone, which is hard and contains fragments of crystals and scales of mica 2118: in other places there are irregular concretionary masses of a hard ash-coloured siliceo-aluminous stone, with conchoidal fracture 2119 & abounding with small scales of silvery mica. Where this stone recurred there were numerous irregular veins partly filled with fibrous gypsum. (c) — This stone, although here occurring as a concretionary mass, I have seen it of a darker color forming whole masses. — in the same manner the white Breccia containing concretions is itself sometimes concretionary. — Different varieties of the earthy stones pass into each other by this sort of structure; so that a pale variety will ultimately only form veins between great irregular rounded-polygonal masses of a darker kind.

In one place on the coast for an extent of 1/4 of a mile, at the same level with the above rocks; a more earthy sort occurred; this either was contain marked with waving water lines, & being amorphous contained numerous rounded pebbles, from the size of an egg to turnip. — They seem chiefly to consist of old altered rocks. — some where were precisely similar to those at Woollaston Isd others were of Basalt &c & a probably from the volcanic origin. rocks on the other side of the bay. —

I am not certain whether is a contemporaneous

211 verso

(a) which partake of character of Slate & Greenstone & sometimes fragmentary appearance

(b) There can be no doubt it is contemporaneous, as so often described on the East Coast 1835

(c ) At Near P. Huapacho I found in a greenish slaty clay. infinite numbers of small interweaving veins, which, exactly similarly as at S.Cruz were partially filled by bundles of gypsum, transverse to the walls of the vein. —

212

1834 July Chiloe (7)

deposit. — but I believe it to be so & that it marks the place where a stream from the higher land, entered the sea. —

There were straight jaspery plates (b), running in a straight N & S. line for considerable distances. — Near Fort Aguy there was much, blackish-grey soft laminated micaceous clay-slate 2120; also a bright & yellow compact earthy stone 2121, both these generally underlay a coarse breccia. —

All these substances were formed into nearly horizontal strata; at one place I found a dip of about 5° to the East. — I believe from a similarity in nature that these beds belong to the same order of things which formed those above the lava (a):

they probably are of rather a posterior date, subsequent perhaps to the first elevations of hard rocks of the south shore. — Do they overlie a mass of lava? How are we to explain the greater elevation of the land south side of the harbor, & the non-continuity of the streams of lava (5 to 700 feet thick) to the Peninsula of Lacuy, excepting by supposing that a greater force elevated it & that the beds bf the Peninsula of Lacuy were subsequently formed & elevated.

No organic remains were present, it is impossible therefore to know the age of the beds; the mineralogical nature 7 veins of gypsum (so common on the other side) renders any great antiquity highly improbable. —

212 verso

(a) I think it highly probable that a considerable part of these beds is owing to dust &c thrown out by volcanoes in the year dust fell near on the greater island to the depth of one & 1/2 inch thick; the decks & rigging of a vessel 70 miles in the offing was covered with the same substance. —

(b) 1835

These veins are not very straight, they consist of several & very irregular thin plates of agate, which unite & separate in an irregular manner, forming with a ferruginous stone the vein. — The agate is smoothly mamillated in some places on its surface. —

I also found one vein filled with regularly crystallized Carb: of Lime. —

213 Chiloe (8)

Near S. Carlos, there is a modern conglomerate which overlies the fragmentary mass of volcanic rock, together with pebbles of lava & older rocks there are rounded lumps of an imperfect coal or lignite 2094, which gives little flame when burnt. — As these pieces are numerous & only occur at one part of the coast it is probable there is a bed of this liquid not far off beneath the sea: it must hence underlie the lava. —

The conglomerate occurs within tidal action, is cemented together probably by same means as that at the C. [Vasos]. —

This conglomerate has most inappropriately been called a "crab mine". —

The first time I walked along the shore on the Peninsula of Lacuy, I was surprised to observe that the rock soft stone was perforated by numerous holes of a Pholas, above the reach of any but a spring tide; grass even was growing close to these & below them.

The shells were in the holes, but their superior extremities broken off; the stone, & consequently the shells, appears to have had its surface worn away to the depth of one or 1/2 or 1/4 an inch.

I judge of this from the form of the hills & included shells. The beach is here steep. — On mentioning the above facts to Mr Williams,1 Captain of the Port, he states that during the last four years, the sea has sunk or land risen for about four feet. In the above period Chiloe has suffered from a severe shock of an earthquake. Capt Williams does is

1 Possibly John Williams Wilson (1798-1857), English sailor who entered the Chilean Navy in 1824.

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214

1834 July Chiloe (9)

not aware that the change took in level took place at that time, but thinks it gradual. I do not believe a change of four feet could take place suddenly without being instantly noticed by those who have boats on the spot. On the point, on which S. Carlos stands I found further evidence, a few feet above the line of high water

there are many small caves, which could only I think have been formed by the action of the tides, these are now separated from their influence by a bank covered with vegetation. — I will describe one of them.

[sketch] level of sea A

The diagram is a section. The cave occurs at the bottom of a cliff: the entrance is about 20 feet above (A) line of high-water, but the bottom of the cave is not more (measuring by rough spirit level) than 8 or 10 feet; the bank or front is merely a talus of earth, with plants growing on it & in front of it; at the bottom of the cave there is grey sand with fragments of crabs & shells; a handful of which could not be distinguished from that on the beach.

I observed similarly-situated caves all along the coast; these occur in any exposed part of the harbor: the fact holes of the Pholades were seen in a sheltered creek, west of Punta Arena. (between 3 & 4 miles from the caves). A change of level a has indisputably taken place within a very few tear, if we suppose the tides to

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215

1834 July Chiloe (10)

have altered, this harbor is so close to the open sea, we mu a similar change must have taken place in the Pacific. —

It is more probable the land has risen

The facts observed at the town, seems comm seem to be the result of the above change, together with a silting up of the beach. — It is difficult to ascertain how much is to be attributed to each of these causes. — The expression of the land gaining on the sea & vice versâ, is very ambiguous. — thus I was told by one person that quite latterly he thought the sea was again gaining, because he judged from the width of a path. — It is probable, that the sea in this case only removed, the horizontal extension gained by the last elevation.

I have observed very generally, that the line of high water tidal action is a little way in front (at A) of the foot of a cliff about 20 to 40 ft high:

[sketch] B high water
B — A — C

Will this small intercepted piece be cut away, & the point (A) move to (C)? Has the whole cliff been thus formed?

In many cases the cliff is 800 or more feet nearly perpendicular; but generally the land at the height pointed out slopes backwards

Does this mark a period of long quiescence? or of some great elevation?

Seeing at the same time a modern formation 700 ft above the sea & the land now rising, one naturally asks has it attained all its elevation to such almost imperceptible rising, as we now are witnesses to? —

One fact, which I have observed, gives some

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216

1834 July Chiloe (11)

little support to this latter supposition. Almost in every part I have observed noticed a thin covering of rounded pebbles, which belong probably to the rocks of the Andes; these occur at all elevations; they resemble in size &c such as now are found on the beach. At the present day pebbles only occur on the beach; I see no reason for supposing it otherwise during the intervening periods of the different elevations. — Granting this vague analogy it requires almost every part of the Island to have been remained once within the tidal action. — On any other supposition (b) (putting aside Diluvium) why directly after stratified muddy deposits & before the elevation comes a bed of shingle (a) to cover the nakedness of the new land? —

The worn irregular outline of the land offers anything but an objection to the above conjecture.

When walking about the peninsula of Lacuy, I was forcibly struck how exact, how identical the form of the land is with a horizontal section of any low range of hills in England.

here there are no valleys, for arms of the sea take their place, & to my eye, are in the proportion, figure & size (c), which a horizontal section of the valleys in any hills composed of soft substances would show. If by successive elevations the bay, & the water which separates Chiloe from the mainland was laid dry

216 verso

(a) These words are applicable to the shingle bed on the plains of Patagonia; there however several reasons make some other supposition necessary. — For, when a tract of land was elevated between the Andes & the Atlantic, has when how did the immense body of shingle travel from its source to the sea. — It must have been spread out, previous to any elevation & only subsequently remodeled. —

(b) No one can suppose such an irregularly formed Island existed in its present form at the bottom of the sea & was then covered coated with pebbles. — The shingle bed looks however like an aqueous deposit. — (as does the section of a beach)

(c) Hence the Inhabitants have scarcely even path-ways, but move about in Periaguas or canoes. —

217

1834 July Chiloe (12) (p 182)

& I for the first time had seen it; it would have been with great repugnance that I should have allowed that the heads of all the valleys main & side valleys in the highest hills had once formed quiet little creeks & coves; that where road might be traced that the f.

That these hills of soft materials had formed the outlying islands in a great & boisterous ocean. — Reflecting on parts of this island & in a lesser degree on the Falklands, has been to me most instructive in this history of valleys. —

No doubt the modern formations in Europe were once skirted by intricate islands, such as these, but the number of years during which they have remained quiet would efface & remove these outlying skeletons, of what the elevations continued would form the hills of continents. — We see this land during the very period of its birth. —

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