RECORD: Darwin, C. R. Geological diary: Lacuy. Chiloe. (1.1835) CUL-DAR35.310-318 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, 5.2011 further corrections by Gordon Chancellor 2.2012. RN2

NOTE: This document continues DAR35.288-303 and DAR35.304. See a map of Chiloe from Narrative 2. Darwin's time in Chiloé is described in the Beagle diary pp. 280-6.

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

See the introduction to the Geological Diary by Gordon Chancellor.


310

1835 Jan: Lacuy (16)

[continued from DAR35.304] pretty & curious. Inspection will show that there is always a symmetry between this arrangement & the lines of fissure. — sometimes these lines determine the centre of the curves, but generally separate one group from the other. at others they bisect the tops of an undulating line. — In another locality rows of small hard sphaerical concretions of the calcareous sandstone bordered on each side the line of fissure. — Now both of these cases prove the age, which I gave to the fissures. — Is the agate &c which we almost see blend into the side of the fissure, a posterior infiltration? Or is it not rather a secretion? We see the intimate connection of the fissure, with the chemical as well as mechanical structure of the sandstone.

The sandstone contains large pieces of black carbonaceous wood: I also found on the beach very much silicified wood (2518: 19) 2518: 19; but (fragments large one about 20 inches square). into but only one in its original site imbedded in the sandstone. In one specimen the wood in patches is particularly converted into iron pyrites (2543). — It is probable the agate plates & silification of the wood are processes closely connected, & the latter fact renders probability to my conjecture that the plates are rather a secretion than an infiltration owing to open fissure. — The fact of the sandstone containing lignite & silicified wood connects this formation with that of the East coast; a fact in itself probable. —

Entering the harbor of S. Carlos, the strata on north part of the peninsula may be seen to be highly inclined; a fact so entirely absent in all the tertiary strata, I have yet seen, as to be worthy of a particular examination

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311

1835 Jan: P. of Lacuy (17)

cleavage

At P. Huechucucuy (V accompanying tracing) we find all entirely composed of mica slate, such as frequently described in all the country to the South 525 ft.

r observ. the direction of the laminae is not always very manifest but where best seen is within a point & half of N & S. the dip is either to W 17 S or Easterly. — The angle various. — I believe the west coast is equally formed of this. — the slate near to the tertiary strata is coated by hard conglomerate of angular fragments of the slate & pebbles of volcanic rock. — At a little point a mile to the East we first have a good section of the softer beds. — V. Chart. —

There is nothing remarkable in their mineralogical nature consisting of alternations of the sandstone & hard slaty clay: the layers are very excessively numerous, quite parallel, distinct & of uniform thickness. —

[sketch] A B C

There are two little hills about 200 ft high, which are faced towards the beach by A cliffs. — Here the strata are seen close dipping at about ∠15° to directly opposite points. — the dip in (B) is a trifle the most inclined. — The external form of the land is conformable to the dip, as represented. — The strike of the beds (or apparent line of elevation) runs N 6° W. — It may be observed that this line & ridge of the point is parallel to the neighbouring headland of mica slate of P. Huechucucuy. —

The two hills with the anticlinal dips are separated by a valley, perhaps 300 yards wide. —

From the configuration of the country, this valley is the

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312A

[sketch]

(x)
Diagram K G

P. Tinuy almost worn away. —

312A verso [blank]

312

1835 Jan: P. of Lacuy (16)

drain of but little water. I should apprehend, this is an excellent example of a "valley of elevation" according to the nomenclature of Prof: Buckland.1

Constantly proceeding Eastward: we find far less regular dips. They appear to diverge from part of a curve. which curve is nearly the form of the coast.

[sketch] NE

The NE dip being for a quarter third of a mile the prevailing one.

We then after meeting some little horizontal have a westerly dip: this occurs in the line of the Huapacho Shoal, & probably forms part of a line of elevation. —

The geology in every respect of P. Tenuy & Huapacho is very curious. — At the latter spot in an E & W section (diagram K) we have horizontal strata of a white friable aluminous stone (such as 2516) covered by the usual strata. — A very little to the westward on the side of the whole headland we have strata dipping at 16° to the West. & form of land — There are here three little points which give as many excellent sections. — The part with this dip is separated from the horizontal strata by a little valley. — The former land being a trifle higher than the latter. — Seen from the sea, looking Southward, we hence have the appearance separated in the sketch (x). — or by joining the diagrams γ & K; bonâ fide however these do not occur quite in same line of section. —

I will now describe the mineralogical nature of the two of the above sections. — In diagram (γ) (100 ft high?) 150 ft ∠r observ: on the East side there is a mass of numerous regular alternating beds of brown sandstone (2514) 2514 & green slaty clay. — They preserve during their whole dip by same thickness. — On the W & lower

1 Buckland 1826 theorized that some valleys were formed not by denudation but by the elevation of the strata from a force acting below.

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313

1835 Feb: P. of Lacuy (17)

down, close to the sea, we have a nearly amorphous mass of a brecciola, much of which is of the nature so frequently described with bits of white stone; the upper part passes into a remarkably white friable aluminous substance (2517: 16) 2517: 16; This somewhat resembles the great upper bed of the Patagonian formation (a). There are a few lines of sphaerical concretions of calc: sandstone not colored in diagram & others few of stratification parallel to the dip of other beds. This white bed (as shown) caps the sandstone & then becoming to the E gradually soft & red & ferruginous (specimen 2515 is intermediate in character) 2515 is divided into numerous regular layers. Moreover the whole of the upper part of Eastern strata are thus stained with the iron. —

The junction of the brecciola of greenish slaty clay is very remarkable. In the lowest part the strata of the latter are continued by an irregular line of variously shaped, but elongated rounded pieces. I know not whether certainly to attribute them to mechanical or concretionary origin. At (b) a layer terminates & is involved in the brecciola exactly in the manner drawn. —

In another transverse section (δ) about 400 yds to the South or strike of beds; the termination of the greenish clay is effected by a mass of fragments penetrated by the brecciola. —

Here, above the white friable aluminous stone we have repetition of the sandstone &c layers: their dip is not however quite so regular. Also, in this case to the E the strata are all in upper part stained with ferruginous matter. —

313 verso

(a) Does this mineralogical resemblance, joined with veins of gypsum. (Salt at Huafo) concretions point out that the formations of Patagonia & Chiloe are of same age. —

I Do not believe the organic remains will oppose this idea. — It is easy to imagine; that the same substances proceeding from the Andes would form similar beds, although in different oceans. —

314

1835 Jan: P. of Lacuy (18)

I presume the two foregoing curious junctions are (the appearance of the which in the first instance is rendered singularly deceptive by fissures which resemble faults) are owing to currents bearing brecciola & white aluminous stone (a) which replaced the sandstones &c. when yet soft (b). — The form & position of the point (b in γ) is however very problematical. —

These white amorphous masses in the three sections are seen exactly in the same line, which is that of the direction of the beds; in a like manner in each case on the East side the strata contain much ferruginous matter. These abrupt changes following a straight line, through a considerable thickness of layers are very singular. —

The position of the white stone, beneath the brown sandstone, where they are horizontal, which us contrary to what we see in the nearest sa section of the inclined beds at (γ), perhaps may be explained by supposing that universally they were once covered as we see at (δ), but that the inferior sandstones beds are only visible where there has been an upheaval. —

I have said that the dip is westerly; the exact strike is in every part N by E, this differs two points from what we have met with at the anticlinal headland: — If we suppose, a line of upheaval: it must be of that class, where the strata only on one side are tilted.

[sketch]

Again proceeding to the E of the horizontal strata of Huapacho, we soon meet with a small west dip, which is continued to the base of a N & S line of low hills, which form a point & reef. The ridge is very narrow

314 verso

(a) That currents did exist, seems certain, from that class of stratification, which is often met with in sand or small gravel, being present in the sandstone: where I d less inclined layers cover or include & cut off others with a much higher dip. —

viz. — [sketch]

(B) In all such cases of these & in those brecciated rocks where fragments are of the same nature as the matrix, a violent earthquake disturbing the bottom of the sea during the period of deposition has probably been a very effective cause. —

Lonsdale1 once remarked to me no case

1 William Lonsdale (1794-1871), soldier and geologist, Curator and Librarian Geological Society of London 1829-1842.

315

1835 Jan: P. of Lacuy (19)

steep & not more than 60 ft high. — There are the regular alternations of sandstone & clay. — V diagram E. — I have stated at the base the dip is West & the angle is about 12°-15°. — on the crest it is in one point 30° & in another 40°. — This latter is the highest inclination I have anywhere seen the strata tilted. — Immediately below the 40° dip & to the E we have instead of an inticlinal line one, a small one of 10° yet to the West; this is at low water mark. —

A short distance (less than 1/4 of a mile) to the South & situated with respect to the ridge, as seen in the Diagram we do meet an East dip of 15°. — Here the strata had abrup passed into the state of gravel. the lines of pebbles showed the dip. — Hence the anticlinal line must run along the bottom of the creek: which we ought to be called a "creek of elevation". —

I must here state (& subsequently discuss) that between the East dip & the high 40° dip, the layers were curved & in one part dipping, to various points at low angles, as in shape like a grand saucer. — the dips converging. — .

Again to the Eastward of this, is the last locality, at P. Huapi Lacuy we have a westerly dip. — Highest point in Huapi Lacuy 240

Thus we see in an E & W band of country 4 miles wide, the strata furrowed by several N & S lines of disturbance, three of which are very evident.

These have manifestly determined the figure of the northern part of the peninsula & explain a greater irregularity of superficies not than generally is observed in the other parts of Chiloe. — Seeking for a continuation of these lines, to the South, in the harbor

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316

1835 Jan: P. of Lacuy (20)

of St. Carlos. I see at the head of Chevereas creek, there is a small East dip. — at a point which is exactly in the strike of strata from P. Tenuy, we have the usual alternations dipping to W 17 N 10°-12.

Further on however the strata resemble immense inverted saucers dipping to SW & even South. —

This saucer stratification is somewhat similar to the great spheres mentioned at Bahia in the Brazils.

When we consider; the extreme regularity & uniform thickness of the layers; the high angle of 40° observed in one locality; — we must come to the conclusion, that strata in parts horizontal or curvilinear round certain arises have been tilted by lines of upheaval:

What these nearly parallel lines of elevation may be owing to perhaps analogy will point out. — In Christmas Cove. P. Tres Montes. we have seen streams of lava & sedimentary beds, which have many close points of resemblance with those of S. carlos, have been traversed & tilted to a considerable ∠° by dykes: There the dykes were parallel running N 34 E:

May we suppose that here there is a system of grand ones, far beneath the surface, which run about N & S. — When a volcano is choked up, after pouring forth many streams of lava, the crust of the neighbouring country is more likely to be torn by injected dykes. This is what has taken place at Tres Montes & is imagined to be the case here. — It is true, no dykes have been met with in the small part, which I have examined. — In almost

316 verso

(a) A part spheare composed of concentric layers, will when horizontally intersected give both either the converging & diverging dips.

[sketch] diverging converging. —

317

1835 Jan: P: of Lacuy (21)

every geological question; facts of an opposite tendency can be brought forward. — It is right to state that occasionally a doubt crossed my mind; whether the cause, whatever it might be, which formed the curvilinear saucer like layers (which do not appear the effect of violence) & the anomalous NE dip to the East of the anticlinal point, may not have acted in parallel straight lines; or whether the strata are modeled over steep points & ridges (like that of Huechucucuy) of mica slate. — The changes in mineralogical nature in lines parallel to that of elevation at P. Tenuy is very puzzling. — The more I reflect on the subject, the more probable does it appear that my first explanation & most obvious explanation of mechanical violence is the right one. —

This band of troubled country, amidst the almost otherwise universal horizontal strata, calls to mind the famous line of elevation in the Isle of Wight. In the latter the great inclination variety of beds, & length of line of elevation claim a far preponderant interest. Here the number of parallel lines, & the exemplification of three different sorts of upheaval; viz anticlinal; the one side horizontal; the both sides with same dip with the greatest inclination — being in the middle are most worthy attention. —

I believe an inclination in the strata is a far more uncommon occurrence here than in England. —

After writing the foregoing part of these notes I read Phill: & Cony account of Isle of Wight:1 till doing so I did not understand the third case. According to my hypothesis, we must suppose the underlying dyke in the place of being vertical is inclined at a considerable angle. — At Lowes Harbor

1 Conybeare and Phillips 1822. GC

317 verso

[sketch] Z
dyke
Laminae of mica slate

318

1835 Jan: P. of Lacuy (22)

we have seen (a) a dyke of 60 yards wide parallel both to the N & S line of cleavage & to its inclination, hence the dip was about 45°. — V. diagram to the left: —

At the short distance to the South, where an Easterly opposite dip has been noticed we may suppose the dyke became vertical (trusting the confinement of the laminae of slate) & tilted equally the strata on each side. — or, as at (Z), the ridge we have described may be an undulation on one side of a regular anticlinal upheaval. —

Both here & in the Isle of Wight the external configuration of the land has been influenced by these disturbances, for in the latter a valley is described as extending between the horizontal strata of the Southern part & the central ridge. —

We know the chalk has been cut by dykes, may we conjecture that beneath the Isle of Wight there is a grand one (such as those 200 yds wide at Lowes Harbor), directly beneath the surface?? — is there another beneath the valley of the Weals?? Are these supposed dykes influenced in their direction by the nearly E & W cleavage of the clay slates as seen in the corn West part of England??? V. De la Beche p. 159 —1

Owing to parallel S & W faults: Yet we may well imagine injected dykes in these lines. —

1 Possibly De la Beche 1831.

318 verso

(a) I do not suppose the whole ridge is more than 120 yards wide.


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