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Darwin Correspondence Project

To David Forbes   18 November [1871]1

Down, | Beckenham, Kent.

Nov. 18th.

My dear Mr Forbes.

I hope that you will have the great kindness to illuminate me on one point; & forgive me for being so very troublesome. I wish much to learn how far the disintegration of rocks goes on beneath a continuous bed of turf.—2 Disintegration, I presume, mainly depends on some one or more element of a rock being chemically attacked; the remainder cohering with little force, & falling asunder.— Am I right in supposing that the disintegration of exposed rock chiefly due to the action of Carbonic Acid dissolved in the rain? If so, not only would some of this water with the acid be carried through the turf; but it is known (Prof. Sachs) that the roots secrete much carbonic acid, so as to charge distilled water with it, & to corrode polished marble.3 Besides this, would not humic acid, generated in the mould, be apt to attack oxides of iron in the rocks?4 I have noticed that red sand lying beneath the Peat, has all its colour discharged.—5 The decay of the roots & stems must set free some Carb of Potash & soda; wd. this act on rocks?6 The decay of the nitrogenous element of plants & of any worms or insects &c. wd. generate some ammonia, which I imagine, would act on rocks, as the air in Stables is said to corrode glass. From these several causes, especially from the quantity of Carbonic acid in mould, I imagine that fragments of rocks within reach of the roots of plants, & surrounded by decaying vegetable some animal matter, would be more liable to disintegration than rocks exposed only to the atmosphere.

Now I should be very grateful if you would tell me whether you think this probable or true; without wasting your time in entering into all my details. If you think so, would you permit me to quote your judgment?— Of course in the colder countries frost would act much more on exposed rocks than on those under the turf; but then does frost disintegrate rock into fine matter, or only split larger fragments into smaller ones?

Pray forgive me for thus troubling you, but I could not think of any other good authority to whom to apply.—

Yours vy sincerely | Ch. Darwin


The year is established by the relationship between this letter and the letter to Daniel Oliver, 15 November 1871.
CD’s query relates to his study of earthworms and the denudation of land. See letter to Daniel Oliver, 15 November 1871 and n. 1.
Julius Sachs. See letter to Daniel Oliver, 15 November 1871 and n. 2.
Humic acid is a complex of acids formed through the microbial decay of plant matter.
CD performed a similar experiment with earthworms, showing that sand coloured red with iron oxide was mostly clear after passing through the worms (see Earthworms, pp. 240–1).
CD refers to potassium carbonate and sodium carbonate (a salt of carbonic acid).


Earthworms: The formation of vegetable mould through the action of worms: with observations on their habits. By Charles Darwin. London: John Murray. 1881.


Inquires about the effect of turf covering on the rate of disintegration of rock.

Letter details

Letter no.
Charles Robert Darwin
David Forbes
Sent from
Source of text
DAR 185: 111
Physical description
4pp & Adraft 2pp

Please cite as

Darwin Correspondence Project, “Letter no. 8075,” accessed on 10 April 2021,

Also published in The Correspondence of Charles Darwin, vol. 19