RECORD: Darwin, C. R. Geological diary: Lowes Harbor. (1.1835) CUL-DAR35.319-327 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. 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. Lowes Harbour is in the Chonos Archipelago.

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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.


319

Lowes. Harbor 8th. [January 1835] —

All the rocks are mica slate (like 2489), with great layers of quartz, as before; which alternates & passes into phyllade ampellite (2488), & a green talcose slate (2487) without order. — the two latter varieties being the most abundant. —

Laminae. slightly tortuous but dip with great regularity ∠ 40°-50° to WSW 1/2 S. —

319 verso [blank]

320

Lowes Harbor 9th. — (6

These are cliffs of tertiary formation, about 300 ft high: composed of blackish hardened mud, with various sized scattered pebbles mostly rounded, some only partly so. —

Exactly same as at E. coast of Chiloe. There were beds of even, thinly laminated sandy=clay, also as at Chiloe. — I found one layer of loose sand, with particles of comminuted marine shells: mineralogical nature of which very different from the true Tertiary strata begin with [dislocations]. — The upper bed almost composed of pebbles in lines: also some small subordinate plains of do. —

On the beach. Block of scoricae. —

The mica slate chiefly with plates of quartz, also much ampellite dip to W by S. about

320 verso [blank]

321

Lowes H. (7

∠45. — The mean with yesterday WSW. — (NB. 2 or 3 or 4 miles East of ship): — At point the mica slate was traversed by dykes & masses of crystalline am rocks in a most extraordinary degree. — The word example simile will more truly explain appearance, than a thousand explanations. it is the thin crust of a tart broken in & the fruit & syrup mingling with it. — The cleavage of the gneiss has had a great influence in the form of the dykes: hence where best seen they are not vertical but inclined. — The sizes junctions are quite distinct, but contorted in every direction; veins & masses of the rock of dyke from the fringe has penetrated the slate, & fragments of the latter are entangled within the dyke: The cleavage is bent & twisted dips in every directions amongst

321 verso [blank]

322

Lowes H (8

those pieces, which appear to have been lifted up on the surface of the melted rock. — In other places layer are quite parallel to the slate & even have a few lines planes parallel to the slate; close inspection will always show, that the dyke fills up irregularities & sends off little obtuse veins, where the qualities of two rocks to the last point of contact are quite distinct. — The parallelism will in a short distance be suddenly quite destroyed. —

[sketch] vertical section
dyke slate S

oth

There is some pyrite in the schists

322 verso

Fig. 2 Horizontal section

9 ft dyke
S N
The included slate with usual dip

x
N
dyke

323

Fig 2 represents dyke about 90 yards wide, with (x) bed 7 yards wide of slate thus traversed & cut off; also (z) mass of slate, traversing these bands parallel to cleavage of the dyke-rock. —

The system of dyke is very large, & forms a chain of islands & low hills: from resisting decomposition. — There is first one of 60 yds, there is a creek of water, of about 100 80 yds wide, perhaps contains at bottom slate, we then have the 90 yds dyke & beyond this there is another smaller dyke. These have same direction as far as this can be made out, which is same with cleavage, or perhaps 2 or 3 points to the East of North. How does this differ from an axis of elevation? (a) it is same in nature with the granite range

323 verso

(a) The band of disturbance in which the schists might only be considered as entangled pieces is at least more than 200 yds wide:

324

Lowes H. (10

[illeg]

Yet the cleavage is only affected in the fragments which absolutely adhaere to sides of dyke. — We here see the mechanical effects of cleavage. — I suspect the tendency of upburst ran NNE & SSW. — The rock of dykes varied much in nature: the changes were in no determinate order: a fine greenish black (conch-fract) base with crystals of feldspar is the commonest (2489): there is much euritic rock 90: which passes by every shade into a white & variegated rock 90: 91 is also abundant, as is 92 with numerous grains of quartz: 2493 is not very common & is a beautiful white trappy feldspathic rock with crystals of do grains of quartz & specks of pyrite: there are others splintery hornstones: the order expresses their comparative abundance in dykes: they occur in great angular masses: —

324 verso

Mellamoy.1 an active volcano. — regular cone, rim of broken craters; looks table topped from some points. —

Peleña (four tops, not volcano. —

There is other great cone to South of Mellamoy. —

1 Melimoyu.

325

Lowes H. — Jany 10th. — (11

Examined some states a mile to the W of Tent point. — which from dyke Isd makes a distance of [blank] miles; the in nearly all of which the dip is exactly the same — here it is WSW ∠40°. — Therefore argument not upheaved. — Rock here chiefly this green chloritic slate — found bed parallel to cleavage of a mixture of chlorite epidote & white feldspar (2498) well crystallized in some specimens: this passed into the schist & gradually assumed the laminated structure. —

325 verso [blank]

326

Lowes Harbor. Jan. 15th. — (12

(day of sailing)

To the westward. —— Usual, WSW dip; some nearly vertical. — transitions from one rock to the other generally gradual. — Saw some layers of regular white feldspar. —

Where enormous veins were freq of quartz, running in irregular direction, were frequent: cleavage contorted as if by violent intrusion:

Yet the quartz in places rather formed lens-shaped masses, than veins & in other places was intimately connected with the surrounding: this was well seen. where the mica slate. contained numerous crystals, placed in waving lines of feldspar? which formed, from weathering a curious external surface:

at [little Potalos] Isd mica slate ampelite in regular. laminae, with but little of the red patches of crystals, dipped W by S. 30°.

326 verso [blank]

327

(13

was traversed by dyke of greenish. homogenous feldspathic rock (2511): which to the depth of several inches was turned red. —

Fig.3

[sketch]

rather irregular
cleavage NNW NE by E C E
WNW A B
F X D

Their form of dyke shows that originally there was the line of fissure AB & CD, which were stretched open by injected stone: E & F gave way: & dykes became united, craters of each following its own coarse or fissure. —


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