RECORD: Darwin, C. R. Geological diary: Anna Pink Harbor / Patch Cove. (12.1834-1.1835) CUL-DAR35.277-285 Transcribed by Kees Rookmaaker, edited by John van Wyhe. (Darwin Online, http://darwin-online.org.uk/)

REVISION HISTORY: Transcribed by Kees Rookmaaker and John van Wyhe, editing by 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.

Editorial symbols used in the transcription:
[some text] 'some text' is an editorial insertion
[some text] 'some text' is the conjectured reading of an ambiguous word or passage
[some text] 'some text' is a description of a word or passage that cannot be transcribed
< > word(s) destroyed
<some text> 'some text' is a description of a destroyed word or passage
Text in small red font is a hyperlink or notes added by the editors.

Reproduced with permission of the Syndics of Cambridge University Library and William Huxley Darwin.

See the introduction to the Geological Diary by Gordon Chancellor.


277

(1) Decemb. 31st "Anna Pink Harbor." Patch

Ascended the grand range 2000 to 3000 feet high, which forms the headland of North part of P. Tres Montes. —

At the North foot, there are some low islets separated from the grand chain by an narrow arm of the sea, 80 to 100 ft deep. —

On the South side there is another arm

These are composed of stra gneiss. (where layers of quartz are separated by broard band of black micaceous rock: & feldspar very rare: (2459). —

) the parallel & regular layers dip to the North; the angle varys; 20° being about the general degree & some are horizontal

fronting this low escarpement, a grand solid mass of granite dips comes abruptly down into the water: The granite is white with distinct & largish crystals: —

In ascending this mountain built of granite (2450 ft high) we twice saw small patches (section A) of the gneiss adhaering to the face of granite. The dip was small & as at North. —

277 verso [blank]

278

2)

These patches were interlaced with veins of ferruginous, quartzose granite. The granite in the neighbourhead was also thus affected. —

But generally it was extremely massive & compact, & without a line of separation. I afterwards on a grand scale noticed lines running parallel to the range. — These hills very barren most singularly contrasted with the almost universal forests of the West coast. (a) —

The summit is broard & is capped (as shown by transverse & longish valleys) by the gneiss to the depth of about 400 to 500 ft. —

This dips as before from 10° to 20° to north.

The gneiss (as shown in section A) extends over to the very edge, above the steeper southern slope of granite (b). Section (B) is a little a few hundred yards more to the westward, where the hill (in the line of range) is rather lower & rather narrower. — Here the granite is

278 verso

December 31st: — 1834

"Anna Pink Harbor"

(a) The hills have generally a rounded form but here & there crowned by finger shaped m points, which must be the overlying gneiss.

(b) The angle of inclination with the sea must be great: V. Chart

279

"Ann Pink"

much exposed; it is manifest it has been upheaved in undulations; for in a narrow gully between two ridges (40 & 50 ft high) of granite, there was a patch of gneiss in situ, which dipped as before & directly on the granite: a little further to the westward (section c) & lowering as before — we have the granite as at (B): but on its southern edge there is some little gneiss. — I traced the junction of granite & gneiss for some distance, & it ran in a straight line (E by S 1/2 S. &c) within half a point of the grand range of the mountains: The junction was firmly cemented, but beautifully distinct specimen, show (A 2460) this; but unfortunately the granite in this one specimen (a) happens to be very quartzose to contain little micah & not of its

279 verso

(a) The line of actual junction appears to be quartz on the part of granite. —

[sketch]
gneiss
dip of gneiss 30°
N granite 1 S
granite 25°

280

4)

usual character: The gneiss then covered the southern a slope of granite (or line of junction) which dipped was inclined to S & irregularly on each side of 20° from 10° to 45°. —

The gneiss had its usual dip to N: the pl spot where I measured it & specimen came from, line of junction dipped at ∠ 25° the gneiss at 25 30° & of course directly on it. — This gave a very singular aspect to the rock junction of the two rocks rocks; the upper angle of intersection being 55°. — Innumerable & most beautiful sections might be seen; little patches as big as half a crown were adhaering solitary to face of granite. — the granite occasionally sent small vein or dyke into gneiss.

Hence we see on the northern base, on northern shore on summit, between the folds ([sketch]) of granite & on its southern edge an immutable direction, (but variable angle) of dip. — If the layers of gneiss, had

280 verso [blank]

281

5) Ann. Pink

been horizontal (we will say like strata of sandstone) could the a mass cone of melted granite upheaved with a highly undulating outline, have given one uniform direction of "point of" dip to the tilted strata. Surely they would have been anticlinal. (a) — My old supposition leads me to conjecture, that the direction of cleavage & upheaval were similar, & that the angle has been only altered: — This perhaps would explain the northern slope edge of hills being more protected by gneiss than the southern, because if it has a dip to north, this elevation would uplift on the South North side more less easily separate rupture the laminae.

(b) ([sketch] conjectured dip). —

281 verso

(a) I am unable to give any better excepting the improbable one that there was an original no dip.

(b) However this may be settled there can be no doubt an enormous mass melted granite has broken through the gneiss, & that the greater part of this has subsequently been removed. — here presents rounded solid forms. — although a contrary supposition is possible.

(a) One is tempted to forget the great NNW & SSE line of cleavage on the coast & to suppose imagine an original northerly dip, & this might explain the greater duration of the gneiss on the N. than S. slope. V last page

We ate sure the granite in a melted form has actually burst through the consolidated gneiss gneiss, possibly when beneath the ocean & when caught entering by great bed of gneiss, & that present elevation may be owing to horizontal movement.

The most legitimate conclusion is that there was a small dip to the West in cleavage, & then an a tilt in line of ESE 1/2 S & WNW 1/2 N would make strata incline on N. side of to the North

282

6

I have said this range of hills & it is a very principal one runs ESE 1/2 S & WNW 1/2 N.

But other & more general line, especially to the North is appears to be at st ∠°. viz NNE & SSW.

It is very remarkable the cleavage following this strange line of hills (excepting on the above supposition of common horizontal strata) when far to the North & South it runs at right angles to it. — It may however be remarked a small West dip elevated by ESE, WNW line, would give a north dip. — but great ∴ it has rather larger ∠° than tilt towards the North. This was on the South slope the strata plains would dip more to SW.

(But I do not understand how to unite the two motions. —

with greater & with greater ∠° the anticlinal line original cleavage.

With this conclusion we must explain section (B & C) by considering the hillocks of granite as having flowed between fissures of the gneiss. —& that the upheaval has been of that class where strata of only one side of fissure is elevated ([sketch])

282 verso

We see the form of land on summit is shown to be owing to form of underlying granite & this is in the transverse section which is slightly undulating. — In the longitudinal section of range the hills are

[sketch] a a a

this, hence probably the granite is undulating in this section & the trans valley gaps in crest (aaa) have not been formed by degradation. —

A conclusion drawn from curvatures in quartz of Falklands

(a) There can be little doubt, that the high range of hills transverse to mica slate of Midship Isd is of same geological nature. — It is very interesting thus at last finding mountain chains (& dykes) decidedly transverse to the cleavage. ؟ Do the outlying volcanoes belong to this chain or to laminae:

The transverse chains prevent parallelism of coast line with cleavage. —

I incline to latter??

283

7) "Patch Cove." —

The dykes & elevations would point out that volcanoes were belonged to this system: the N & S trend of coast, that & line one with other & analogy of Andes, would lead to the contrary. — Moreover, in Chiloe we know do not know of any transverse line:

Perhaps one belongs to one & other to other class of facts. — I suppose from Chiloe, that there have been elevations parallel to cleavage. [sketch] There would appear to be a parallelism in dykes & mountain chains: & connection of dykes & volcanos has been shown. —

And again dykes (at Midship Bay) & grand transverse granitic range. —

All which would show connection of volcano, with these ranges. — but not in Chiloe

283 verso [blank]

284

8) Jan 2d. — 1835

Crossed the range, more to the E. but failed in finding the gneiss. — Whole South side to the coast of the Estero is granite. —

The lines of fissure clearly run nearly vertical, ESE 1/2 S. & WNW 1/2 N: Parallel to the lines lines dykes, there were 2 or 3 broard bands of eurite (2461). porph: with fine octagons of quartz & with feldspar: the rock has a slightly laminated structure: I was in doubt whether there were beds on dykes, but finding the 2 rocks preserving each the respective characters in contact, & that the line of junction is not quite regular, I have no doubt of its being a true dyke; perhaps of same age as the fine-grained quartzose ones which have bee mentioned as cutting the gneiss. — In a narrow gorge, perhaps 800 ft deep: one of these dykes could be

284 verso

[sketch]
B 2 1 E C B
sea A A

AA. — section of valley & its lakes (1 & 2)

E divortia aquarum

BB. one of the bounding cliffs. the sides being almost perpendicular & quite inaccessible. —

C. Dyke of Eunite, which is seen on each side of valley. —

NB. Slope AE not above 1/4 of real proportion. —

285

9)

traced on each side to about the height of 400 ft. which clearly proves that this much at least has been removed & my theory of point of elevation too much considered. — Valleys on each side of range meet on crest & form gaps: these gaps are generally flat & not knife edged: there is great difficulty in imagining their formation excepting by the agency of the sea. — The above dyke, although not in the line of divortia aquarum yet attained a higher elevation, & which in the case of that gorge proves it has been excavated by some method. The quantity of degradation on the very ridge of any range, must can be scarcely of any consequence:

How could a stream run at its source in the lake (50 ft below divortia) (1) remove a great circular mass of hard granite: The sea might. —


This document has been accessed 2581 times

Return to homepage

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

File last updated 2 July, 2012