RECORD: Darwin, C. R. Geological diary: Chonos and Tres Montes. [12.1834-1.1835] CUL-DAR35.233-258 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/).

REVISION HISTORY: Transcribed by Kees Rookmaaker, corrections and editing by John van Wyhe, corrections by Gordon Chancellor 5.2011. 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 Marginal notes are here integrated into the text.

Darwin was in the Chonos Archipelago and Tres Montes 14 December 1834 to 14 January 1835. These places are mentioned in the Beagle diary, pp. 274ff and in South America, pp. (119-120), 156-160

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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.


233

1) Chonos & Tres Montes

The Islands of the Chonos Archipelago & the outer coast of the peninsula of Tres Montes are constituted in a similar manner, as the Islands of Chiloe. In this case the primitive being even more preponderant over the tertiary as in Chiloe the tertiary is over the old crystalline slates. — In both we have a small volcanic formation. —

I will just describe the tertiary strata, in the few localities, where I have seen them. — The Island of Huafo rather belongs to the group of Chiloe than to that of Chonos. — it has an elevation of 800 ? feet. — is composed of blackish fine grained compact muddy sandstone; there are some layers of much flattened concretion of hard grey calcareo-sandstone. —

There are no pebbles. — The cliff in many parts was marked by bands of immense numbers of Turritella (2537 ... 2542), with these were Fusi Venus (the species with concentric lines found in E. coast of Chiloe) & striated [Athecanthus]:

There were specks of lignite. — In a E & W section & on East side of Island, a very small dip to the East might be perceived. — There were some large caverns: these were very dry, & the insides thickly coated by a great quantity of a pure white saline substance, in an highly uncompressed form (1143). — These caverns were probably formed when the sea stood at a higher level; in the same manner, as at the present day, it excavates on the beach curiously shaped passages &c in the soft stone. —

Lowes Harbor. Lat 43° 48'. There are cliffs about 300 ft high composed of what generally be called alluvium. — it is an hardened blackish mud, with many variously sized pebbles, some quite rounded, others only a little so. — There were beds of fine-grained, laminated sand-clay. — In short, exactly similar to so many sections seen in the East coast of Chiloe. — I found one layer (near base of cliff) of loose sand, in which

233 verso

R.N. p. 7 dikes many kinds1

R.N. p. 36 ascertain carefully whether there is much calcareous matter in strata of Chonos. — Valuable point in metamorphic history

Daubuisson mica slate character of, p. 802

1 References to the Red notebook.

2 Aubuisson de Voisins 1819.

234

2 Chonos Archipel.

were many comminuted particles of marine shells. This mass of (so called) alluvium rested on the mica slate of the surrounding district; its upper parts were composed of gravel arranged in horizontal lines. — there were also some small subordinate places entirely composed of pebbles. Both these latter facts have been frequently observed at Chiloe. —

Isd of Lemoos Lat 44° 30'. Appears entirely tertiary: strata of fine grey sandstone similar to that of Chili (2469: 69) calcareous: scales of mica: even laminae: there are lines of comminuted shells. — Here & there are placed irregular mamillated concretions of hard calcareous sandstone. — There are also layers of smaller ones which affect very various shapes; such as bones, jugs, eggs, pebbles & intestines (2470: 71). — I noticed the Venus of Huafo & Chiloe with concentric lines, but could not extract it, also a few others (2473). — As is so generally the case, the shells, especially the perfect ones were chiefly contained in the hard the calcareo-sandstone concretions. — What is the cause of this? Has the calcareous matter determined the presence of the shells? or has its solidifying power only preserved them?

I incline to the latter supposition. — We must suppose the concretionary structure to be formed at some depth within the deposit; yet a in an irregular concretion which was 8 or 9 inches in diameter. a lamelliform corall (2472) seems to have encrusted it. — I do not understand this, many parts of the strata have been removed by currents, at the epoch of deposition us exposed a concretion just formed? —

The sandstone contained patches of black glistening lignite & a very few quartz pebbles. — Judging from the fragments on beach, there must be a covering of these strata of agglomerated gravel. —

In one or two views I saw that form of out line which bespeak the

V. Diagram on back of page

234 verso

[sketch] A

[sketch] B

A & B. Island escarpements in lower plains: —

235

3 Chonos Archip

quiet residence of the ocean at a different elevation from the present:

This formation extends some way to the East & North, where it becomes mingled as in Lowes Harbor with hills of mica slate: The large island of Estero is appears entirely of the same as Lemoos. — For some degrees to the South, we see nothing but hard rocks; in the gulf of Penas behind Xavier Isd the chart places clay cliffs. — & I understand shells were extracted from them. — There can be no doubt from the similarity in mineralogical nature; in the few organic remains, & in the form of land, that the formation of Chiloe & Chonos is one. — What its age may be perhaps will be difficult to determine. I should be inclined to think somewhere corresponding to the plains of Patagonia. — It would seem probable that in the Chonos each of the detached Isd have never formed part of one plain, but that each owes its origin to a nucleus of primitive rocks. Thus in Huafo on East side we have an East dip. Whilst on the West coast of South part of Chiloe the plains (which probably composed to the strata) incline to the West. —

It As in Chili we chiefly find organic remains in the finer sedimentary deposits further to seaward, those nearer to the mountains are too coarse either for the existence or for the subsequent preservation of shells. — With respect to period of elevation I have no data: on the outer coast of Tres Montes in Christmas cove )Lat 46° 35') there was in a several places a beach of pebbles & shells 20 feet above high water mark; also a single Fissinella some hundred feet up in the woods. From what we know in Chiloe, there can be to no doubt about the recent elevation of this coast. — (a)

I will now begin with the primitive rocks. I call them primitive: solely because they are chiefly crystalline micaceous slates: I may remark

235 verso

(a) I see in Byrons Narrative of the loss of the Wager1 the following remarkable passage: "But we thought very strange upon the summits of the highest hills were found beds of shells a foot or two thick."1 This in latitude from 47°-48° South & corresponds to those parts of the plains of Patagonia, where the evidence of a recent rise in the land is most palpable. —

1 Byron 1832, p. 45.

236

4 Chonos Arch

that all these rocks from the north point of Chiloe to Pen. of Tres Montes kn are so extremely similar in every respect, that they unquestionably belong to one formation. — Beginning at the North. —

At Lowes Harbor (43° 48') (we will first describe each place separately & subsequently discuss the two principal class of facts, viz. concerning the dykes & cleavage of the slates). The rocks are mica slate, where broard plates of white quartz are separated by much dark, micaceous mineral. (such as 2459). This may serve for the description of all the mica slate in this paper & that of Chiloe: for the only variety consists in the fineness or frequency of the alternations of the two minerals.

The mica slate here alternates with & passes into Ampelite (?) kernels of quartz with feldspar (2488) & a green [Taleaceous] chloritic slate (2487). Indeed these are more abundant than the mica slate. — The ampelite frequently has little patches of an yellow orange mineral. — The alternations are without any order. — I found one large bed, parallel to the planes of cleavage, composed of chlorite epidote & feldspar (?) with mica layers (2498). This passed into the schists & gradually assumed their laminated structure. — In one place small crystals of feldspar were arranged in undulating lines (2512); & in another regular layers of white feldspar was included in the slate. — Every where there many & very large veins of quartz: These in this form seemed as if injected & it is remarkable that the cleavage in their proximity become disturbed. — Yet from the mineralogical connection with the containing rocks & lens-shape of many of these veins, they must be of contemporaneous origin. —

The cleavage was very strongly marked, the extreme variation in point

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237

5 Chonos Arch.

of dip was from W 11 S. to W 27 S. — West 20 S. may be assumed as the general dip. — The angle was much more regular than what is usual being about 40° in all places. — I walked in E by N & W by S (line of dip) for a distance of four & 1/2 miles; in nearly the whole greater part of this, the the rock was visible, & in every where the dip was as above stated. — Now this is an instance of what Baron Humboldt has observed on a larger scale. No one will suppose, that this coast line is a section of a mountain about 24000 feet high entirely composed of alternations of mica, & chlorite & ampelite slates. Yet if the regular planes of cleavage had ever been horizontal, this would seem the almost necessary conclusion. —

At a point 3 miles East of the anchorage the mica slate is traversed to a most extraordinary degree by grand dykes. One simile will better explain the appearance than a thousand descriptions; it is that the crust of a tart, broken in by the pressure of the knife, & where the syrup & fruit represent the rock of the dykes. — The cleavage of the mica slate seems to have had a great influence in determining the form of the dykes; hence they run in an irregular N & S. direction & are not vertical but inclined to westward. —

The junctions of the two rocks is quite distinct, but the line of it contorted in the greatest degree: veins & masses from a main dyke will have penetrated the slate & the fragments of the latter are entangled within the dyke. The cleavage of the slate in immediate contact is twisted & dips in every direction, as if carried up on the surface of the melted rock, but a few yards further off it is regular. In some parts, the laminae

237 verso

[very faint] [illeg] Parallel at plains of [illeg]

238

6 Chonos Archip:

of slate have been separated & quite parallel layers of an extraneous rock been injected. — Close inspection shows their nature of violent origin of even these apparent layers, by We see small wedge shaped veins entering circling the slate from the layer of its direct junction this being joined on either hand with unformable masses. — Fig. 1 shows one such vertical section. — In Fig. 2 we have a horizontal section of the central part of a dyke 90 yards wide, where there are included great masses of mica slate, which retain the usual direction of their cleavage, so that they have not been much displaced. = (x) in a bed 7 yards wide which in this traversed & obliquely cut off. = (z) shows the laminae in 3 lay lines injected =

The mineralogical nature of the dykes is subject to much variation; & the changes occur in no determinate order (NB. not being able to describe the rocks I shall scarcely say anything about them: an accurate examination of the whole series which traverse the schists of the Chonos Archipel. will be interesting in many respects) a fine greenish black (conch; part:) base with crystals of feldspar is the commonest (2489): this passes by every shade into a white variegated kind (2490): 91 is also abundant: 92 with numerous grains of quartz: 93 is not very common, is a beautiful white waxy feldspathic rock, with grains of quartz & pyrites. — There are some splintery hornstones. — The above order expresses the comparative abundance of the varieties — They all occur in large angular masses. — The system of dykes is broard, about 250 yards; in this mingled mass of schists & amorphous rocks, one dyke can be distinguished about 60 yards wide & another of 90 yds. — As I have said the

238 verso [blank]

239

7 Chonos Archipel

direction is about N & S: or North & a little Easterly: perhaps this line results from the mechanical effect of schists acting on a line NNE & SSW line of upburst. — The dykes form a line of low hills & little islands: partly from resisting decomposition, & partly perhaps from being an elevated line. — Indeed what is this but a line of elevation? surely such would be its effect, if covered by yielding sedimentary strata. — With the surrounding slates, this does not seem to have happened. —

In a small outlying Island, there was a dyke, the form of which appeared to have been determined by lines of fissures in the mica slate. Fig. 3 — The CD & AB were are these lines & E & F the sides, which have given way to form the dyke. The melted matter instead of continuing the injection of the two separate ones dykes, after intersection, has united itself. —

The points (B & D) show the lines in which the dykes would have run if the pressure had not been removed sufficiently relieved by the existing dykes united one. — The rock is a greenish fel homogenous & feldspathic (2511). This externally to the depth of several inches partially decomposes & turns red. —

To the South of Lowes Harbor 43° 48' as far as Midship Bay 45° 18' Lat a distance of more than 100 miles, Mr Stokes collected specimens for me: 11 miles to the ESE we have a chloritic slate with epiolite (like 2498): which dips to the East. — Everywhere else we have the micaceous schists (& fine (2498) ampelite Lat 44.13') dipping to the West. —

(Mr Stokes did not observe to a point, but the dip was universally westerly, he proceeded along the outer crust; seldom being more than 10 miles from it. — He also remarks the general occurrence of large irregular quartz veins both in the mica slate, & ampelite & green Taleaceous slate. —

239 verso [blank]

240

8) Chonos Archipel.

Midship Bay now called. Vallenar Lat. 45° 18' — The rock is coarse mica slate; the micaceous substance between the quartz, often passing into Ampelite. (2378: 79). — The schist is crossed by many dykes, in a space of 1/3 of a mile I met 5. They are all composed of nearly the same substance, viz. massive pale-grey granular greenstone, containing few crystals of feldspar, little iron pyrites & grains of quartz. (2380: 81). — The two largest, 9 & 7 yards wide, ran in N 65° E direction from N 32°-65 E°.

There were lines of fissures parallel to the dykes; which & the latter were traversed by faults at nearly right angle, occasioned by a slipping in the direction of the laminae. —

The almost universal dip is to W 25° S at ∠ 40°: I saw some vertical & where so the dip was more Southerly. Hence the cleavage is at right angles with the line of dykes. — In the low islands the line of hills & creeks are parallel to the cleavage. — But a mile little to the Eastward there to is a bold range of mountains, from 2000-3000 ft high (∠r measurement, which I feel sure are of granite.

I judge from color, form & vegetation as compared to separ a known range some distance in Tres Montes. — This range was appears to run in a N 56° E NE by E & SW by W (V. chart?) direction. Which does not differ much from the line of dykes. —

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241

9) Chonos. Arch. & Tres Montes

Ynchemo Isd

Ynche Isd. — Lat: 45° 48' (North of Tres Montes) is composed of a grey quartzose rock mica, feldspar, rounded [gr] of quartz (؟. not pure quartz?) which contains grains of quartz (2442). or it passes into a finer grained, semi=conch fracture. semi-feldspathic rock (2443). ؟ It will subsequently be shown how closely this rock is connected with allied to the micaceous schists. — It is not laminated, but divided by very many extensive, & smooth planes of fissures: these intersect each other at an ∠ of 45°. — Hence we have on the outer coast & this is a striking feature on the outer coast of Tres Montes many & regular smaller pyramids. —

A. Pinks Harbor (a few miles SE of last place) — We have here the usual mica slate, which in one place passes into ampelite; its dip is not very manifest where most regular it was N E 11° N giving nearly the usual direction but opposite dip; where more inclined & less regular it dipped more northerly, even in one case to E 56 N: — In a vice versâ manner as at Midship Bay.

[illeg] more inclined planes dipped more Southerly. —

I examined three dykes & I believe saw others: the 1st is composed of a white pale brown feldspathic scale of mica with octagons & quartz, base (2466) or a more granular one (euritic?) (2465) with numerous regular octagons of quartz & crystals of feldspar: This dyke is 8-10 yards wide & may be seen running for some length at rt ∠ to cleavage of slate, viz.

241 verso

De la Beche Translat of Ann des Mines of 1767 (no folding of strata about gravel.1

This is applicable to my theory of elevation of mountain chains

Granitic veins discussed R.N. p. 88
change where approach overlying rock

[sketch] W 28 N

1 De la Beche 1824.

242

10 Tres Montes

N 45 E: the 2d dyke had a similar direction & formed a line of islands & low hills is composed of a greenstone with abundant white feldspar (2467): the 3d is a small & not very regular one, runs in a N 68 E line & is composed of a blackish, fine-grained greenstone (2464). —

Patch Cove. 45°: 52' (a little to the WSW of last locality) Is situated at the northern base of a high range of hills of granite. — In front there are some low islets formed of the usual mica slate (2459): the laminae dip to the North. the angle varys 20° being the most common: some even being horizontal: the lower escarpement is separated by a narrow channel 80 to 100 ft deep from the bold, almost praecipitous slope of granite.

In a like manner the range on its southern base, is bounded by a narrow creek. — The granite is distinctly crystallized: feldspar white. mica black. (a) occurs in grand rounded masses on a large scale I see these are found The range is from 2000 to 3000 ft. high, it runs E 28 S. & is the principal one in this district, forming the northern part of Peninsula of Tres Montes. —

In ascending one of the peaks 2540 high, I twice passed over small patches of the schist, adhaering on the face of granite (V. Diagram section A). — their dip was small & to the North. — These patches were traversed by veins of ferruginous quartzose granite; & the vicinity the granite of the mountain was equally affected. The summit is broard & is capped by the gneiss to the depth of about 400 feet. The dip, as before is from 10°— to 20° to the N.

The mica slate, as seen in the section extends over to the very edge over the steep southern slope. — I crossed the range, further to the Eastward, to its southern base, but saw no schist at all. —

242 verso

(a) occurs in grand. rounded compact masses nearly bare of vegetation & frees from the interminable forests of these countries; — viewed on a large scale there are lines of fissure which run parallel to the range of mountains. —

(b) The inclination of the mountain, which we ascended must be very great V Chart

243

11) Tres Montes

Section (B) is a few hundred yards to the westward of (A) where the hill (in line of range) is both rather lower & narrower.

The granite is hard, much exposed; it is manifest it has been upheaved in undulations transverse? to the range; for in a narrow gully between two ridges 40 or 50 feet high we find a patch of mica slate dipping as before & directly on the granite. — Still further to the W & lowering as before (Section C) the mica slate coats the southern crest. — I traced the junction for some distance it ran in a straight line. —

E 17 S. — that is within a point of the grand range of mountain. The junction was precisely similar to that of a greenstone dyke, with its walls: it was firmly cemented, but beautifully distinct. —

Specimen (2460) shows this, but unfortunately in the once piece (a) the granite has not its perfect characters. The junction was rendered the more striking by the large angle it formed with the laminae of slate. At the spot, from whence I broke off the specimen, the granite sloped at angle 25° to the South, the mica slate dipped as before to the North at 30°. — Hence the lesser angle of intersection is 55°. — I can feel no doubt, that the range of mountains has on been a great mass (an enormous dyke?) for what is the difference with the 90 yd dyke of Lowes Harbor excepting the greater size of melted rock which has been injected, in into & upheaved the superincumbent mica slate. The mica slate originally must have coated the whole mountain; probably the period of injection took place when the whole mass was beneath the pressure

243 verso

(a) Little patches, as big as half a crown, all with their proper dip are yet adhaeres to bare face of granite. The granite sent out from beneath small wedge-shaped veins into the slate. — A strong presumption of its injected igneous origin. —

NB. It is highly probable that some granite such as those which occur in beds between gneiss & mica slate may hold the same relation to this granite, which the greenstone of Chili included in the breccia porphyry does to the greenstone of dykes (on summits of hills, viz. Campana of Quillota)

244

12) Tres Montes

of a deep ocean. — More than the present elevation can be easily accounted for by subsequent horizontal upheavals. —

[sketch] D north south

We thus see the laminae of slate have an immutable point of dip (but variable angle) both at base, N. side. summit between folds & on southern crest. It at first appears strange, that the injection of the granite has not affected the slate with anticlinal folds; we must suppose that the present undulations in out line of the granite were originally wedge-shaped fissure in the slate injected with the fluid stone. — Vide D.

The immutable dip is to the North, therefore the direction of the cleavage is E & W: on the whole extensive line of coast we constantly see it nearly N & S. — I cannot suppose it to have been originally different here; but rather, that it has been altered by the protrusion of the granite.

If we suppose the laminae to have had a small dip to the west & that they were tilted by a line of upheaval. (range of hills) E 28 S; on the northern slope, we should have a new dip to the North, & on the southern to somewhere about SW & with a greater angle than the original cleavage. —

(I do not know how to compound these two motions). — This is what I believe to have taken place.

[sketch] E m. slate S N granite

From the North dip being found even on the very southern crest: the upheaval could not have been anticlinal; but rather as at (E). Where North side is most tilted. — The I have described this hill particularly, because I have no doubt, that it is the type for

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245

13) Tres Montes

the whole range. — We see the same general rounded outline, but here & there the summits are crowned by finger-shaped projections, owing without doubt to the capping of mica slate. — Although this range & perhaps a few others run E 28 S. — Yet more to the northward, the general line is certainly much more to the northerly, viz E 60°-70 — or at rt angles to this line. —

In the centre of the granite range I found some dykes: I have said lines of fissure run parallel to the range: These perhaps determined the direction of the dykes, for they likewise have run in bearing E 28 S. There are three (perhaps more) broard ones; are very similar to one described at A. Pinks Harbor, being composed of an euritic base with fine octagons of quartz & crystals [illeg] of feldspar (2461): the rocks has a slightly laminated structure: the walls of the dyke are not quite regular, but the junction is very distinct. These dykes are of great length, I saw one showing its course (for 4ooft) up each side of a gorge,which is perhaps 800 ft deep. —This leads me to reflect concerning the origin of some of these valleys. — We have seen in a transverse section in the outline of the granite is not seen:

This probably is the case to a greater extent in a longitudinal section of the range: But we see, from this instance of the dyke, that great part of the transverse gorge have been excavated out of the solid rock.

245 verso

[sketch]

S B A 400 ft C E A B N

section half way up mountain

AA Section of the valley

BB. — do of bounding hills on the side

C. Eunitic dyke, seen on each side of valley & probably continued far beneath in distant line.

E divortium aquanum

1 & 2 small lakes.

245A

N.B. The patches of gneiss mica slate in B & C are not part of same line, but parallel ones. —

ESE 1/2 S Range

[sketch]
dip from 5°—20° to N. —
North South
A dip A
B B
Sea C C

[colour indication] gneiss mia slate dipping in all cases from 5° to 20° to the North
[colour indication] granite

[calculation] 19 [+] 28 [=] 47

(A) Highest & broardest part of the ESE & WNW range

(B) About 200 yd to the West, lower & narrower WNW 1/2 N

(C) Still about 200 yd to West & still lower & narrower. — perhaps 400 ft below higest part

245A verso [blank]

245B

[water colour sketch]
W: N
S. Dyke running N & S
N
x
E

Fig: II

Horizontal section of part of the 90 yard dyke; with included masses of mica slate, retaining their true position, as shown by the cleavage. —

The band (x) is about 7 yards wide. —

245B verso

Fig. I

Vertical section of bed-like dyke parallel to cleavage of slate; but connected with an uncomformable mass. —

[water colour key] mica slate

[water colour key] Injected rock of dykes. —

246

Name Section Cleavage Direction of dykes Mountain chain
Christmas Cove. Latitude
46°:35° Head of Harbor —
Little central Island judging from a distance
(NB. To the South, the cleavage all appeared to dip W 22S.)
┴° running N 11 W Dip W 11 S
dip W11N: ∠ 45°
7 or 8 dykes all running exactly N 50 W
one ran N 67 W
Hills (& line of creek) N 34 40 W
Dark Harbor Latitude some miles of coast
S. Estero
∠° various from 20° to ┴° dip to W 22 S   [calculation] 90 [-] 68 [=] 22
Patch Cove
Latitude 45° 52'
dip North owing to granite range N 18 W
28
W 22° N
WNW & SSE
Granite range of its line of fissures.
N 68 28 W
NB. to the North ranges appeared to run N 22 E at ┴° to [North]
A. Pink Harbor: Lat:
few miles ENE of last locality
Dip E 11 N small ∠°. (where ∠ is greater dip even E 45 N) N 68 45 E
2 dykes SSE NE
a third N 68 E
 
Midship Bay Lat 45°:18'
whole district
dip W [illeg] 25 S
NB. near granite range
2 dykes N 65 E,
others varying from this to N 31 E
Granite? range running N 68 56 E (?)
Lat. 44°:13' dip westerly } Mr Stokes
44.7 do
44.5 do
43.56 dip Easterly
Lowes Harbor
Lat 43.48 Whole district
W 11 ... 27 S N & S  
Chiloe whole South extremity: great district W. 22 S.   NE & SW line of hills ??
Chiloe. Central E 11 N    
Chiloe. North west dip & E    

246 verso [blank]

247

allude to mica slate at P. [Wreckeray] passing into angelite

S
30°
A A
N

Strike of cleavage

N. 11 W
N. 11 E disturbed?

N 22 W

E & W. disturbed

N 11 W

N 25 W

North. for 100 miles of coast

N 19 W

N 22 W
11 miles of coast

Northerly
Central & North Chiloe

247 verso [blank]

248

14) Tres Montes

The Estevan dyke did not occur at the Divortium aquarum, but some way lower down to the S. If this had been the case, it would have been almost impossible to account for this gorge by the agency of running water: As it is, there is much difficulty, because the dyke is seen on each side, higher than the point (E) or Divortium aquarum (V. Diagram). — I believe the sea to have had a passage through the gorge, anterior to present elevation. — Neither is the circular expansion of the valley, flanked by praecipitous granite, seen at the little lake (1 & 2) easily explained by agency of running water. — On the other hand, by small coves are of frequent occurrence on a sea coast. —

Dark Harbor. Lat [blank] (Port Stephen) S. Estevan. Examined about 2 miles of an East & West reach of coast. — The rock is a grey quartzose micaceous schist (2437); which often contains numerous some grains of quartz & occasional scales of silvery mica feldspar crystal (2441). This variety is not to be distinguished from the rock of Ynche Isd (2442). — Therefore we have a mineralogical transition from the semi-feldspathic rock (2443) of Ynche to the true micaceous slates of this harbor. — The formation is here very regularly laminated & the point of dip ranges within a few degrees of W 22 S range N 22 W: the angle is not constant, perhaps 40° is the most constant, have seen saw it as low as 20° & again nearly vertical. — The formation is traversed by innumerable veins of quartz. — It is manifest that this formation is identical with the more perfectly characterized mica slate of the north:

And from geographical position & mineralogical passage, we may equally feel sure about the relation of the rocks of Ynche Isd.

248 verso

irregular

249

15) Tres Montes

Cone Harbor & Port St Andrew San [Anoles] Lat. 46°45'. — The common rock is a pale homogenous feldspathic rock (2386) (2385) wh. [illeg] specks of quartz & crystals (؟ fine grained greenstone?). — There is a most remarkable conical hill 1600 ft high: from its shape & analogy with the islands about Cape Horn, I expected it would have been composed of greenstone. — I found the rock similar to the above general one; in parts it assumed grains of quartz & few crystals of feldspar & had a sonorous conchs fracture; more generally some was blacker (2387); other parts were paler & had an obscurely brecciated structure (2388). From the general appearance of these rocks, from a similarity with the slates of T del Fuego, when in proximity with greenstone: & with those of Chili included in the porphyritic breccia. I am strongly inclined to think all these are only slates altered by fire. The brecciated structure is one another point of analogy with those of Chili. — These rocks are compact, but yet are clearly laminated, I found the strike of some vertical beds N 11 W. (a) — From the mineralogical alliance, as shown by Ynches Isld & Dark Harbor, & from similarity of cleavage this formation is probably identical with that of the mica slates. — Are then the mica slates, sedimentary slates altered by heat? — This train of reasoning adduces a slight presumption.

The irrigation ranges of hills & creeks of water run N 40 W; they have an elevation from 1500 to 2000 ft; it is not improbable they have a nucleus of granite & many detached fragments: — The outline of the hills have a peculiar obtusely angular outline, which is very similar to the ('so supposed') altered slate of Woollaston Isd.

249 verso

(a) Judging from external form, I believe the same sort of rocks extend to C. Tres Montes: from the ship the dips appear W 22 S. —

250A

[sketch]

N
volcanic form
[illeg]
N 40° W
metamorphic slates
granite
cleavage [illeg] N 11 W
x Lava which has not flowed. —
fine of slate dip to W

250A verso

The rocks porphyry (2413) formed are blackish greenstone (2412) This is remarkable as very [word obscured]
dykes 5 ft were dykes composed of different materials

rather irregular great dyke, which ran composed of a pitchstone with a breccia

dykes all run NW 1/2 W & SE 1/2 E; they are being in size up to so must equal in quantity the field, they [text obscured]

250

16) Tres Montes

At a little island under which we anchored in the harbor, on entering, I saw "altered slates" dipping at about 45° to W by N.

This direction is exactly three points different from the usual one: it is probably caused by the mass of porphyritic greenstone, which forms the eastern part of the islet & seems to underlie the laminated rock. — The greenstone is smoke grey & earthy crystals of feldspar indistinct, green (chloritic?) patches (2382); it is traversed by numerous veins, which blend into the mass, of a finer grained chloritic greenstone (2383). — The relation of this greenstone I do not know, but it will be immediately seen, that in mineralogical nature it is connected with some adjoining fields of lava & dykes. — I do not however suppose it has ever flowed; its position is too much under the altered slate. —

About half a mile to the West of the above formations, there is an extensive volcanic district; the lavas have been subaqueous; much consists of that fragmentary nature, where it is difficult to say whether fire or water has been the cementing agent;

The lavas are of very variable mineralogical nature; their structure is by no means simple; there has been a succession of streams. — the whole has been traversed by very many & large dykes. — In one place, the lava composed solid fields, & had a globular concretionary structure. — Specimens (2405: 06: 07: 08: 09) out of these, the first is more abundant, : 08 passes into 09: Seeing the compact nature of most of these, I was surprised to find the field gradually passed into a fragmentary mass, where highly cellular pieces were clearly united by

250 verso

layer this volcanic structure [illeg] identity

Tuff strata dip to North ∠ 45°
range East & West

251

17) Tres Montes

an aqueous deposit. — At another locality, the commonest varieties (a) were brown stony cellular lavas (2390: 91), the cells often lined with a green coating; less commonly grey ones of similar nature, some of which were of little spec: gravity from extreme cellular structure (2392: 93: 94). — There is also much of a slate-color, quite strong, with very few crystals (.2395): this kind is often laminated (Is not this of frequent occurrence the more strong, the more laminated?). the plates show tendency to globular structure. — Perhaps even a still more common kind is a rubbly sort, which seems related to pitchstone: this frequently forms the external layers of the spares of the former strong kind. —

This pitchstone (2397) often is penetrated in its whole substance by fine white friable lime; a blow of the hammer makes it fly out in powder; it fills up all the interstices; certainly it must have flowed with the melted rock, in the same manner as large quantities appear to have done at St Jago. C. Verds. —

On a mass of angular fragments cemented together, or rather the interstices filled with an aqueous deposit, there were are beds of a brownish, fine-grained indurated sandstone (2401: 02); this contained a few fragments of lava & small bits of black, glistening lignite. — The sandstone alternated scores of times with a soft brecciola (2403). — The layers are parallel & denote tranquil aqueous deposition; above these beds, there was is a neat stream of cellular lava. — All these, dipped dip to the North ∠ 45°: immediately in front & under their basset

251 verso

(a) The lava contains some large hollow sphaeres of agate: one was a foot in diameter: the inner surface is largely mamillated & bright red (2400). — The junction of the agate & lava is curious & does not look like subsequent infiltration (2399). —

Dike which has caused the tilt ranges WNW & ESE

252

18) Tres Montes

there is a great dyke of a brownish compact greenstone (2398). — This at its very edges, becomes slightly cellular & finer grained, so as to be like part one of the fields (2391).

The dyke has an obscure transverse prismatic structure.

On the opposite side of the ravine there appeared I could not reach it) a corresponding mass of cemented fragments of cellular fragments lava. — The dyke runs WNW & ESE; it appears to me certain, that the injection of this dyke has caused the high dip in the overlying scorice, sandstones, & lava:

the band of disturbance is more than 20 yards wide, probably 60 or 100 yd. — Is this a common occurrence?

In the common greenstone dykes of the secondary formation of Europe does it occur to such an extent? Perhaps a less incumbent pressure would cause the strata to yield. = At a short distance from this place, there was a bed of lava which dipped at about ∠ 70° to the South — I do not feel sure whether it is a dyke or thin stream. — It seemed bounded on each side by an aqueous deposit, which would incline one to the latter supposition; on the other hand, in the direction of the bed in a neighbouring cliff there was an appearance of a dyke. — The cells were very flat or lens-shaped; the long axis being in line of apparent dyke [sketch]. Will the fi position of these bubbles tell whether this had been originally a horizontal stream. — If so the disturbance here has been on a grand scale. —

Again I found a little cliff, 12 feet high, composed of many

252 verso

Second mass of [infuceous] layers dipping to NNW at 22°
& ranging WSW & ENE

close & dike ranging North 50 West or NW 1/2 W & SE 1/2 E

certainly there are 3 [valleys] as I [illeg]

see [three words illeg] how far differently

grand scenes of D. [illeg] ranging [p. 59] cutting through [illeg]

253

19) Tres Montes

sedimentary layers, capped by a bed of slightly cellular lava (like 2405). These dipped dip to the NNW ∠ 22°. Close before & almost parallel, there is an irregular dyke, running NW 1/2 W & SE 1/2 E, which sends off small sub-branches & cuts through both aqueous & igneous rocks; it is here evident that this dyke has not tilted the strata.

[sketch] dyke N 22 W N 50 W

But possibly this is a larger branch behind. —

In the neighbourhead there is some cellular calcareous tufa (2404). A specimen which was brought me by Mr Jonstone,1 who informs there is also some conglomerate. This is near Rees Island. — The proportion of the sedimentary beds to the crystalline rocks is scarcely anything; in which respects & in the structure of lava there is a strong resemblance with the volcanic district of S. Carlos. —

The small part of the volcanic formation, which I examined, is chiefly remarkable by the number 7 size of the dykes, & still more so by their exact parallelism. — Besides the two described I found in a short space (1/2 of mile) 7 others transverse to the district.

Their sides are generally straight. Three are composed of fine-grained greenstone (2411) one is 10 yards, another 8, & the third 2. — The porphyry (2413) forms a large one of 30 yards. — Almost universally the rock of dykes is different from the field they traverse. The blackish greenstone (2412) though not like the dike [illeg] is an exception, as it resembles that of the field (2406).

The others were are large dykes, but I had not time to examine them. — All these dykes are exactly parallel, running N 50° W. —

1 Charles Richardson Johnson Mate on the Beagle, see Narrative 2, "Mr Johnstone" is mentioned twice in the Beagle diary pp. 60, 207

.

253 verso

one irregular dike transverse to the others

254

20) Tres Montes

I was first impressed with the idea, that the field of dykes had was were been formed merely by the crust of a stream of lava, fissured, & filled up the inferior & still fluid stone. But the derivations in mineralogical character & alternations of sediment. 9which could only have been formed in tranquil water) with lava streams, renders this impossible. — Besides the true dykes there was a pap, or irregular broard dyke, transverse to the usual direction, composed of a pitchstone with a brecciated structure (2410). —

I have stated my belief, that the old feldspathic rock, through with the volcano must have ejected its lava consists of altered sedimentary slates, yet I do not think this change metamorphis has been effected by the agency of this volcano; the presence of the granite points out a more probable cause. = It is only part of the grand metamorphic action which has reached along the whole of this coast. We cannot however fail to be struck by the nearness of direction of the hills & dykes, N 40° W & N 40 50° W. — also of the mineralogical alliance between the dykes & lava streams. to the mass of rock which seems to have uplifted the inclined layers of altered slate. — In a like manner at Midship Bay there is perhaps a connection between the lines of dykes & ranges of granite hill. — But I do not understand how intimate this connection is. — The volcanic matter has a thickness of at least 200-300 ft; how much more I do not know. — The form of the land speaks nothing; the hills are irregular, water-worn & covered with forest.

254 verso [blank]

255

21) Tres Montes

A few miles to the South, I observed (but took, with other some of the officers no particular notice) a high rounded hill of a different shape from the rest; the summit & one side seemed bare & covered with some soft greenish substance. — I feel little doubt think it probable that this is a volcano, perhaps the source of the above lava streams, but at a period, long since passed. —

It is a subject of some interest to consider to what system this volcanic form belongs. We see it placed, in N & S line (parallel to chain of Andes & nearly to the strike of cleavage) with respect to that of S. Carlos & (Castro ??). They both have been may have formed an advanced line, in the same manner as Villa Rica. Osorno .. & S. mountains now do — but further removed:

They both have burst through the same formation; are similar in their structure & partially in mineralogical nature; are have both been subaqueous & covered small deposits, which probably belong to the same tertiary epoch. — They seem to have determined in each case the extreme west limit of line of coast. —

Against this view some arguments may be brought forward. — In the Chonos Archipelago & Peninsula of Tres Montes all the principal chains of hills run at various angles, but more or less transversely to the line of cleavage & of the Andes. — In this respect they singularly differ from those of Chili, although there is an agreement in neither class being absolutely connected with the main cordilleras: here channels of the sea separate them as in Chili land-straits & plains are interposed. — Also the dykes in a like manner in each particular district preserve a degree of parallelism (perhaps in some cases identical with mountain chain)

255 verso

dikes impossible to [illeg]

256

22) Tres Montes

which but at distances. intersect the direction of cleavage at various angles. — These dykes are mineralogically allied to the lava streams; but in the one instance are subsequent to existence of the volcanic forms. (a) — We Can they the point out to what system of hills the volcano belongs? However this may be — the existence of the transverse granitic hills chains may be brought forward, as an b index of the most probable system. It must be remembered this does not apply to Chiloe, where there are no transverse line but a north & south band of crystalline slates. —

(cleavage)

Before concluding I will make some remark on the subject of cleavage. — The extent over which the it has here been observed is about 300 miles nearly in a N & S (?) line. In breadth but to a very trifling distance. — The whole formation is mineralogically almost identical. — Changes in its nature take place parallel to the cleavage. — I have drawn up a table of the strike: at first inspection the coincidence in bearing is not so strik remarkable, untill it be remembered, that each observation represent every the position of every bit of rock in that district;

That Mr Stokes rough general observations applies to more than a hundred miles of coast. — But chiefly that there are no exceptions. — We no where have a wide difference, excepting in one place where there was an evident cause in the protrusion of a great granite range. — The mean of all my observations (b) give for the direction N. 19.W; in

256 verso

(a) Does this afford an presumption of the age of all the dykes described in this paper? The extreme variety of mineralogical character tells nothing; for we see these even in the one & the same dyke, (also in some lava streams) as for instance at Lowes harbor. —

(b) The line of two outer coast of Chonos & Tres Montes runs about N by E. — The general outer coast including Chiloe about N & S is North with a very little Easting. — In the general small details in form I can see very rarely no coincidence with direction of cleavage.

257

23) Tres Montes

generalise this in closing discussion —

scarcely any instance will there be found a greater variation than a point on either side of this. — To a greater degree of accuracy I do not pretend. — The point of dip whether to W or E is subject to more variation; although the former is far the most general (a). — The angle seems governed by no regular law. — The fact of mountain chains & dykes being transverse to the cleavage has already been remarked. — I have frequently observed in older countries a parallelism of coast line & laminae of the schists. — I believe this will not apply. — When the charts are finished I shall be able to judge. —

The existence of high transverse ridge, would certainly materially interfere with any such coincidence. —

I am ignorant what a figure a curve would show which intersected the meridians at about the ∠ N 19 W in the latitudes between North of Chiloe & South of Tres Montes. This lime, if there were no distances might perhaps be expected to form the coast. — Also, if the W 23 N cleavage of S. T. del Fuego or the W 39° N of Port Famine were produced up to the above latitude, what would their bearing with the meridian be?

[text largely obscured by superimposed paper]

257 verso

(a) ؟ In same manner as on T del Fuego the SW dip preponderates over the NE? —

This maceous schist formation is part of the general base of America — belongs probably to the gneiss of Chili. —

What is the nature of the outer coast between C. Tres Montes & Sts of Magellan? — South of this latter point Desolation Isd is said to consist of granite; which, as we see further to the S. in Beagle channel X, perhaps has been or is the nucleus of crystalline slates. — May this be considered as the constitution of the primordial range: which for a long period being an active line of volcanic agency has at various times poured forth trappean or lava rocks & metamorphosed subsequent sedimentary deposits?? —

X Capt King states that on East side of Clarence Island there is much mica slate, & greenstone & that S. Desolation is more granite

The number of dykes shows that beneath the whole of this primitive formation, there has been a stratum of melted matter which when ejected has a different mineralogical nature. —

I have used the expression primitive to this formation, from having no proof to the contrary. — I suppose from characters drawn from the constituent state nature of the rocks it would rather be transition.

[text largely obscured by superimposed paper]

1 King 1832.

258

24)

When the charts are completed the outer coast from North part of Chiloe to C. Horn will have a very similar configuration. —

Dikes successively formed, yet are parallel to ranges of mountains & creeks in San Andres


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Citation: John van Wyhe, editor. 2002-. The Complete Work of Charles Darwin Online. (http://darwin-online.org.uk/)

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