RECORD: Darwin, C. R. Geological diary: Tierra del Fuego. (1-2.1833) CUL-DAR32.85-95 Transcribed by Kees Rookmaaker, edited by John van Wyhe. (Darwin Online, http://darwin-online.org.uk/).

REVISION HISTORY: Transcribed from microfilm by Kees Rookmaaker, corrections and edited by John van Wyhe 6.2010, corrected against the manuscript by van Wyhe 7.2010. 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 pencil was used instead this is noted in the textual notes. Marginal notes are here integrated into the text.

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

See the introduction to the Geological Diary by Gordon Chancellor.


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Tierra del Fuego

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The geology of the Southern part of Tierra del Fuego is chiefly characterized by a great slate formation. — This extends in an East & West direction for about 140 miles, & certainly is continued in the NW for a great space. —
No volcanic action of any kind
Hot Spring &c &c

At Port St. Policarpo the horizontal strata of the East coast end. — from this point & including the whole Southern tongue, the slate commences. — Picton, New, Lennox & Navarin & Mass of Hoste island belong to this formation (I exclude in this general view, interspersed masses of Trappean rocks) The northern part of Wolleston island is slate. as is half of Hoste island. — The slate terminates in the NW by a point, or peninsula as it were, running in between the crystalline rocks; in Hardy peninsula, it gradually changes its character into that of a Trappean nature. From 6 miles, to the East of the bifurcation of the Beagle channel, the whole north shore is slate. — In its mineralogical characters the slate is tolerably uniform; it generally resembles an imperfect roofing slate, varying however in its fissibility. — It often contains Iron pyrites in small veins, as it also does occasionally quartz. — In many places it becomes jaspery, & hence banded with colours, in planes identical with those of cleavage. — I was never able to perceive a trace of organic remains in this southern division; yet, from its general appearance, & especially from the occurrence of interstratified beds of Feldspathic rock, with angular fragments, as likewise of a (Brecciated?) at

Specimens presented to Geolg Soc from Chanticleer at Staten land &c

85 verso

Daubuisson1 on small globular basalt Vol II p 157

On the decomposition of Trap Rock

On the decomposition of Trap Rock so as to appear like Breccia Daubuisson Vol II p 580

Fitton on geology of Tierra del Fuego — — Geolog. Proc. November 1827 -1828

Mr [Bollaut] has talked of rocks at R. Institution from T. del Fuego perhaps slates from P. Famine

V. Back of p. 103 & 104 for more notes

1 Aubuisson de Voisins 1819.

Mr [Bollaut] …V. Back of p. 103 & 104 for more notes] added pencil.

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Wollaston Island, I do not rank it with the primitive rocks. — It certainly is superimposed (a) on the mica slate of the main W N W chain. — The direction in which the cleavage runs, is singularly uniform. — On opposite sides of Navarin island, a distance of 35 miles, I could not with a compass perceive the slightest difference. — It was here vertical planes & running W N W & E S E. — This same range I have noticed in various places in the Beagle channel: it is also common to the micaceous schists. — Although I have remarked great mineralogical changes in planes parallel to the cleavage; yet I think this latter & the stratification to be quite distinct. — In The few places where I noticed stratification will be subsequently noticed mentioned. — In the best marked localities the dip of the cleavage was to the SSW. & at a high angle. — but in others it was vertical & sometimes dipped to the NNE. — Perhaps on the west side of the formation the dip (& range) was more westerly, & on the East. the reverse: This is what might be expected from the general outline of the country. The lofty granite chain 7-8000 ft (b) runs, which includes M. Sarmiento, & other lofty mountains, the general height of which is being about 3800 feet, runs in a WNW & ESE line. — The general form of the slate hills may be divided into two distinct classes. — The one

86 verso

(a) I do not see that the distinction is of any consequence: but I suppose from similarity of cleavage with old rock & absence of fossils; although never alternating with old rocks, it would be called [Orimeter]

(b) This great & central chain includes M. Sarmiento, whose height by Capt. King is 6800 feet; by Capt Fitz Roy 6100; likewise another mountain 4300 feet & general range 3800. — the line which joins the two highest Sarmiento & — ) runs in a WNW & ESE line, this unquestionably being the direction of the chain. — Height by are calculated by trigonometry by Capt Fitz Roy

(a) I do not...called [Orimeter]] cancelled.

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which is most common, includes low rounded & saddle-backed hills. — These are covered with detritus, which favours the growth of the entangled woods of the [Beach]. — The other form is that of a high serrated ridge, where the weathered irregular outline of the weathered slate is grandly picturesque. — As such hills are invariably only those bordering on about 3000 feet in height, we may suppose some mineralogical change: — in the only place where I ascended near to these ridges. — the slate was more fissile & of a less earthy nature. — Nevertheless I feel convinced (a) they belong to the great slate formation.

In Navarin island. there is such a ridge & likewise on the N side of the Beagle channel. — I could not perceive any striking parallelism between the line of slate hills & the WNW range of cleavage. — Yet in pulling in the Beagle channel to the Westwards, the sides looked uneven; whilst on turning to the East view, the eye, as in a street, was immediately guided to the perspective distance. — It occurred to me that as the Beagle channel runs E & W & the stratification cleavage ESE & WNW a westward view would meet all the salient points (b); but on looking in the contrary direction the one hill. would cover the next & so form a continuous line. —

87 verso

In Navarin island I clearly saw a transition from low rounded to the serrated ridges: x I at the time attributed the difference to the ridges being more exposed to the weather & not protected by vegetation. —

(b)
[sketch of hills in pencil] WNW W E
If these are supposed to be separate hills, & the line only showing the original ridge of elevation, it is clear how different an appearance the sides of the channel would put on according to the direction of the pr view. —

1834. Feb: I think this is true: but the valleys of detrition, which enter the Beagle channel at rt angles to it render any certainty about range of hills very difficult: —

[bottom of page] 1834. Feb: (a) To the W of Ponsonby Sound the summits of the hills are certainly slaty. — I do not feel so sure about the Dog Jaw in Navarin Island. & the rough mountains on the opposite shore of the Beagle Channel.

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Proceeding to the West in the Beagle channel at the termination of the slate, micaceous schists of several varieties are met with. — As far as I could observe the (roofing or greywacke) slate lies immediately on this formation. — From the occurrence of numerous angular blocks, it is probable that the summits of this main chain are granite. & that the sides, as we have seen, are mica slate. — Proceeding to the NW well characterized granite is found which at a distance may be recognized by its rounded outline & naked barrenness. — As I have said, the direction & dip of cleavage is the same in the mica & the the more modern slate. — I suppose therefore all to the ranges of hills were elevated subsequently to the formation of the N (roofing) slate: — V. infra

The Western coast, as an inspection of the chart will show, is broken up in every direction by islands & channels. — From the few opportunities I had of observing examining them, they would appear chiefly to be composed of greenstone & other Trappean rocks. — Their relation either to the Granite or the more modern slate is very obscure: the near to the West entrance of the Beagle channel, in external forms & gradation of mineralogical characters, they would be classed with the Granites. But in Hardy Peninsula the various & singular Trappean rocks, would seem to owe their origin to the altered slate.

88 verso [blank]

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The islands of Hermit, Horne & South Wollaston are greenstone. — In Hardy Peninsula I observed some granitic rock surrounded on all sides by the Trappean & Slates. It occurred to me, that perhaps the whole of the Western rock owe their origin to the alteration of & fusion of the slates & that this recurred at the time, when those slates which overlie the micaceous schists were upheaved. — It may also be noticed that the range of these outer islands is parallel to grand central chain. — (B) These remarks are however only vague hypotheses: — In connection with these, the The extent to which the West coast is intersected by water is very remarkable. —
V. Cockburn Channel
I think it is chiefly to be attributed to the original form of the land. — in Hardy Peninsula the irregularity of the hills, if partly submerged would form so many islands. — No doubt the action of the open ocean on the outer coast. must be very great; but then it must be remembered, how completely a chain of islands breaks the swell; the water within the channels & sounds is as quiet as an inland lake, so that even the trees stretch their branches over the salt. water. — Such inlets must have existed from the first & accordingly the water is generally much deeper in them (a), than on the outside crust; for these, the continual weathering of the rocks is daily adding fresh deposits.

89 verso

(a) Capt Cook found in entering Christmas sound the water deepening from 37 50. 60. 170. Such land being elevated, would present a curious mass of alluvium cut through by the retiring waters. —

(B) The older formations will be seen to form to a curved band of nearly equal width [words heavily deleted] & extending over nearly a gradient of own oval. It is in fact the prolongation of the Cordilleras slightly bent. — Its present form is owing to the tertiary strata, which lie at the west Eastern side. — Comparing this part to strata the northern cordilleras, the rocks of the outer coast have a greater resemblance to the central range of those districts, than the Granite & Slate Axis has of the Sarmiento range has. — I suspect, the bifurcation of the Trappean & Granitic axes, I suspect to be owing to the union of the true N & S line of the Andes with WNW line of the clay-slate country. — This kind of attraction is however perhaps purely fanciful. —

NB Those Granite on S. arm of Beagle Channel & in Hardy peninsula & which do not contain quartz, would according to [analogy] drawn from the north be the most modern; although here the older ones form the main lines of elevations & not these recent kinds. —

90A

Sea-Side

Mean H.-Barom
at sea. side

29.316  
: Attached Therm 54.2

Centigrade
= 12.3

Thermometer 53. = 11.6
Bank. Hill
Barometer. 27.734  
Attached Therm 45.° = 7.2
Thermom 42. = 5.5

 

  Mil Metres
Barometer at Sea = 744.7 = 5988.4
at Hill = 704.6 = 5547.8
    440.6
Diff. of attached Thermom: =5°.1 = 7.5
  Apparent height

448.1
433.1

diff of detached Therm   14.8
    447.9
  Latitude 1.3
    449.2

Height in Feet 1472

Banks. hill. Good Success Bay: Terra del Fuego

February ... 1833

Calculated by method in De la Beches. Geology:1

Turn over

1 Beche 1831.

90A verso

Trigonometrical measures given at c/s
Beech trees about 8 foot high
less general = range

Snow less on the Hills—

Sea side Barometer 29.282
6 & 3/4 A.M: Attached Therm: 51°.5
  Atmosphere 48°.5

 

Banks Hill Barom 27.734
9 A.M. Attached Therm 45°
  Atmosphere 42°

 

Sea side Barom 29.348
11& 3/4 A M Attached Therm: 57°
  Atmosphere 58°

Trigonometrical... on the Hills—] added pencil.

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I will now pass on to a more detailed account of the localities, which I was enabled to visit. —

Good Success Bay

The country here is exclusively slate. — in the lower parts the slate is generally compact 878 (a), siliceous & would appear to contain much iron. — Here also Greywacke on a Breccia, where large angular fragments are imbedded in a matrix, of such a similar nature as to render the junction scarcely visible, is found. —

On the mountains the slate is fine grained dark grey 927, very fissile; plates yellowish colour, very smooth (talcose) It occasionally contains in veins iron pyrites & quartz crystallized; there are also beds almost composed of Iron & silica 928; these are generally in lofty situations. —

Cleavage. In Good Success Bay the dip of the cleavage varies more from the usual SSW direction, than in any other place. — It here dips ∴ more WNW to the SSE ∠ 43°; this is in accordance to the shape of the land & as Staten land runs in WSW & ENE line V D. Webster, perhaps it may be a continuation of this slate formation. — A few miles within the country the dip runs to the South. This was the higest land in the neighbourhead. & an elevated ridge divided the course of the streams, on one side into St. Polycarpo, on the other into Good Success bay. It was

90 verso

(a) 1834. Feb: Thetis Bay. between Capes Diego & St Vincent. the rock is all slate, which is very irregularly fisile; & ferruginous, siliceous, sandy. hard compact coarse varieties are mingled together without order Specimen (1858) 1858 is part of a thick tortuous layer in the above slate. — The rock was penetrated by many veins of silex & lime. — The whole mass was so confused that I could not obtain one clip of the cleavage. this was to the S by W at small angle: perhaps ∠ 8° to 10°. — The hills & ridges seem to run in a SW by W & NE by E line, which accounts for the form of the bay; but is different from what I have generally observed. —

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Good Success Bay

remarkable, that this ridge was in a N by W & S by E direction. that is nearly at rt angles to the range of cleavage.
ridges transverse As I have reason for supposing the line of elevation of & those of cleavage to be parallel, it is a proof how enormous the removal of these mountain masses has been. — The summits of all these hills were jagged & praecipitous & I imagine the serrated ridges are formed of this variety of slate, but on a more lofty & greater scale. —

Navarin Isd

Having circumnavigated this island, I had several opportunities of examining its structure. It is generally hilly, but in the northern half there is a high serrated ridge, which perhaps runs in a WNW & ESE direction. — The slate is nearly uniform in its characters 948 942 964; & in many places. jaspery in bands parallel with the cleavage. — cleavage This almost invariably runs in a WNW & ESE line & generally dips at high angle from 40° to 90° to the SSW. — In other cases it is vertical. — I have already noticed a case where at 35 miles distance. but nearly in the direction of the cleavage no difference was perceptible. — In some few places it dips to N by E. at a high angle. One would imagine from this that the cleavage was originally vertical & that disturbing forces had tilted it a little towards either of the opposite points. — I noticed in several places within Beagle Channel. horizontal waving line bands

reason for supposing...greater scale] cancelled.

91 verso [blank]

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8

Navarin Isd

of a whitish colour in the slate. — They result from the weathering of the rock as internally there was no perceptible difference in its nature: These bands are of course quite distinct from the jaspery ones parallel to the cleavage for as the cleavage is nearly vertical, & these are horizontal. the 2 sets of planes cut each other nearly at rt angles. — These bands in form & direction were like those, which dirty water, when it is slightly agitated leaves on any vertical surface. — They must point out some slight chemical changes which run through the rock in horizontal planes & these are perhaps allied to stratification: I ought to have mentioned that there were (irregular) (not very regular) seams or divisions parallel to these coloured bands. —

(Ponsonby Sound) In Ponsonby Sound, at the place, where we settled the Fuegians there are some interesting geological facts. The slate here alternates with beds of crystalline rock. — The slate is of its usual character 942, sometimes fissile, at others more compact: it contains the jaspery bands parallel to the cleavage. X — The beds & cleavage dipped at one place to at ∠ 53° to the S by W 1/2 W. & another to N by E at a high angle, that is within half a point of the opposite direction. — The crystalline rock is composed of a feldspathic base (a), with crystalls of feldspar. it is brittle — highly sonorous, conchoidal fracture & 945 much divided by natural & parallel fissures into Rhomboidal masses. — The plane of these

92 verso

(a) In both slate & the Feldspathic rock, there were oval cavities on the surface, which apparently had contained some soft mineral now washed out. —

In other places I have noticed oval nodules of a darker colour & more compact slate within the slate: —

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Navarin Is

fissure are curious from their perfect smoothness & regularity. — This rock is still more remarkable from containing numerous angular fragments of slate similar to the adjoining beds: in all cases the pieces of slate are united with the Feldspathic rock; but in one specimen (946) 946 there would seem to be a gradual transition from the slate to the crystalline; as if the latter had partially dissolved the imbedded fragment. The alternations of this rock with the slate are exceedingly numerous. so as in bulk to be nearly equal to it. — It occurs also together with slate (948) in the mountains. (a) — In the space of a mile. on the coast, there must have been a hundred alternations. The slate was compact & with jaspery bands; the Feldspathic rock the same as before: with however the exception of two beds, where the crystals of Feldspar were larger & the whole mass more crystalline. 947 it was of a much whiter colour & decomposed more readily. — The beds of the two rocks & cleavage of slate. dipped nearly vertically to N by E. — The beds varied in thickness, sometimes 2 or 3 feet. & again several yards, neither rock was predominant. — The parallelism of the beds & the regularity of the whole formation most uniform. The junction of the crystalline rock & slate. was as fine

93 verso

(a) The slate here dipped to S by E ∠ 70°!

93V

[hand-coloured geological section]

Ponsonby Sound
Fig 1

Slate Feldspathic rock Slate Feldspathic rock Slate

N by E — section — S by W. —

93V verso

P 77 & 78

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Navarin Id

(if it may be so compared) as the best work in inlaying of woods. & as straight as a mathematical line. —

On viewing these rocks & considering the crystalline nature of one; its sonorous properties; its easy fusion into a white gloss; its prismatic cleavage (or fissure), one I was inclined to believe it had been in its original formation, injected in a melted slate. between the slate. —

In support of this, these are the angular fragments of imbedded slate & still more the following appearance. — Fig (2) represents a bed of slate, the lower part of which seems to have been removed by the Feldspathic rock. & in another place (although not near as in the drawing), small pieces of jaspery slate are imbedded in the crystalline rock & placed in same direction as their cleavage, as if just separated from their bed. — Yet with all these arguments in your mind, (a) one view of these numerous alternations, their perfect regularity, the straightness of the junctions, the great extent, yet the narrowness of the slate, &c. immediately does away with the idea of subsequent injection; & the conviction of their being a contemporaneous formation forces itself on the mind. — The appearance of the Feldspathic rock with its imbedded fragments of slate is (I mean not as explanation), as if some power, such as that of Electricity or Caloric, had

94 verso

I am aware to how great an extent volcanic rocks are said to be protruded between strata. —

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11

Navarin Island

acted on a bed of slate during its formation & prod breaking & partially dissecting it. had formed a crystalline instead of earthy result. — The extreme regularity of the junctions will hardly to allow the supposition of successive deposits. & I shall hereafter show, with connection with cleavage, the improbability of supposing imagining these beds to have been horizontal. — (a)

The phenomenon appears to be intimately connected with the separation of the quartz into straight bands. in the mica slate at Monte Video. —

(Wollaston Isd) The mountains on the east side of Wollaston island (perhaps a central small island) are formed of a rock closely allied to the above described Feldspathic rock. — Sunday Bay Here angular fragments of slate & Greenstone are imbedded & blended in a semi-crystalline base. — The rock varies much; in places becoming Feldspathic & crystalline. 1039 ... 1042 — Specimen (1042) is of the latter kind; & (1039) of a Breccious nature; the intermediate numbers. showing a transition. —

chief ridge W EW. X The rock is hard, sonorous. with an angular cleavage: the best marked lines running WNW & ESE. — Together with these were small, irregular beds of a compact slate. —

Navarin Isd Within the slate formation there are several masses & dykes of greenstone & hornblende rock. — These appear generally to have burst through the slate. subsequently to (whatever the cause may be) the SSW dip & hence to

[sentenced unfinished in current document; it is continued in DAR32.98]

chief ridge] added pencil.

95 verso

(a) In the Pyrenees, these would appear to be somewhat similar beds; which according to some authors, it is difficult to believe were once horizontal. Greenough on Stratif:

A parallel to NW Chain.


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