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Darwin Correspondence Project

From Adam Sedgwick   4 September 1831

Tremadoc [Wales]

Sepr. 4. 1831

Dear Darwin

I left Capel Curig the day before yesterday & the stupid red nosed waiter did not shew me your letter till a few hours before I started. Otherwise I should have endeavoured to profit by your information respecting Cwm Idwal.1 I ought however to have seen the madrepores;2 for last wednesday I went from Capel Curig to Cwm Idwal and thence clambered out at the top by the side of Twll Dy a very curious chasm which I suppose you have seen I then scaled the crests of Glider Bach & G. Fawr and zig-zagged down to the Inn. Your information did not however surprise me, as madrepores are quite as likely to be met with as terebratulæ, which seem to occur here and there thro’ ye Snowdonian chain. I found terebratulæ among the talcose slates of Foel Goch a precipice just to ye west of Cwm Idwal— I found organic remains in ye slate of Moel Shabod, but did not stumble on any bed in which they abounded. At first I found specimens about the middle of the great zone of slate, & afterwards in the stone walls above the wood (by the way stone walls are good localities for fossils and often tell us a good story wh. it wd be difficult to make out without ’em). I don’t understand your puzzle about M. Shabod.3 Why should not the rough beds at the bottom, on ye N.W. side, pass under the blue slate with shells? I don’t agree with you in thinking that the mass of trap on ye crest of the hill is under the slate. It appears to me decidedly to be over it— And in ye great Cwm with ye small lake on ye East side we see the slate under ye trap.— Again the trap wraps round in a horse Shoe shape to ye S.E.—twists round to ye E. side of the great Cwm, & then runs in a mass about 200 yards wide in a directn. about N.E. for a mile or two, between two great vertical masses of baked slate. E.G.,


My picture is detestable and out of all gentlemanlike proportions but you must suppose 4 & 5 to be pulled out considerably to ye left hand of X No.1 & No.2 are as in your letter; No.3 ranges a mile to the S—. & then comes round to No.5—the connexion is not ideal, as you may walk all the way on trap from No.3 to No.5. The slate No.4 where in contact is as hard as a flint. I’m beginning to think that I shall not reach Barmouth this year. Therefore have the kindness to address me at Carnarvon, which I shall for some time consider as my head quarters—tho’ I shall probably not often be there— To give you some outline of my progress I will send you a skeleton of my walk.

21st. (this day fortnight) from Carnarvon to Dolbadarn; having in ye morning heard a sermon an hour long & gone a geological Sunday walk towards the S.— 22 d. a hardworking day along the Crown, Shoulders & ribs of Snowdon 23 d. a still harder day, commencing with ye slate quarries. (4 hours before Breakfast, what wd your man have said to that?)—& then scaling Lidir Mawr, & zigzagging along ye Crests of ye Chain to Twll Dy, & so home in ye dusk (24 th. weather bound the whole day (25 th. hammer my way to Cap〈el〉 Curig—lunch, & then hammer my way to Tinny Maes— making anot〈her〉 modest collection of geological specimens.) 26 th. Carnedd David, Carned Llewelyn, & so down the rough crags of Porphyry (we saw at a distance) down to Aber—& thence back to Tinny Maes. 27. ascertain the nature of ye beds below the great slate zone by two or three traverses; & end at Bangor. 28. In a great measure a day of rest—E.G. drive to Aber. 29 th. To Conway by the crests of the hill—thence across the water to ye ridges between great & little Orms head Evg drive to Llanrwst.— 30 th. Cross the mountains on foot by the line of ye lakes to Capel Curig.: caught in a mist and deluge of rain and steer over ye mountains by Compass, reach the turnpike within a quarter of a mile of ye road turning off to ye Inn. 31 Lake Ogwen Glider Fawr & Bach, &c &c to C. Curig. 1st Sepr The crest & flanks of Moel Shabod— Evening excursion to ye knolls N. of the trap &c. 2d. Mountains between C. Curig and Llanrwst, & visit some of ye mines— 3d. Bettws, Penmachno, Maentwrog, & Tremadoc, ascending one or two hills by ye way. 4th. Sunday, a day of rest—continual mist & rain.

My best regards to your friends at Shrewsbury. | Yours most truly A Sedgwick

P S. I saw no basalt at Lake Ogwen but a very black pyritous variety of rock something between Lydian stone & compact felspar. It differs from basalt in being extremely siliceous.— Perhaps I did not see the spot you mention. I am going as soon as wind & weather permit to make traverses between this place & Carnarvon. They will take 10 days


CD’s geological notes of the tour of North Wales are in DAR 5. They have been transcribed and annotated in Barrett 1974, in which this letter and letter from Adam Sedgwick, 18 September 1831 are also published. Professor Barrett’s transcriptions of the many Welsh names in the letters have been adopted with his permission. Cwm Idwal is described in CD’s notes (pp. 13–14), Barrett 1974, pp. 159–60.
Ibid., p. 160.
Ibid., pp. 160–1, but CD’s notes make no mention of a ‘puzzle’. The statement ‘the Slate appears to overlie the Trap’ appears on p. 161.


Reports on his geological work in N. Wales since he and CD parted. Answers CD’s queries.

Letter details

Letter no.
Adam Sedgwick
Charles Robert Darwin
Sent from
Source of text
DAR 204.5
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 116,” accessed on 23 June 2017,

Also published in The Correspondence of Charles Darwin, vol. 1