To J. V. Carus   23 March 1876

My dear Sir

I am not surprised at your puzzle over the passage which ought to have been made clearer.¹ Every reef consists in large part of agglutinated fragments & coral sand; and when I spoke of the vertical thickness of the reef I included the basis supposed to be formed of such matter, which if the reef were upheaved would be distinguished with difficulty from the upper part. You might make the sentence clear by adding after the word reef “including the basal part formed of agglutinated fragments & coral sand together with some of the more delicate deep-water corals”²

D^r Allen is a misprint for Allan at p 68³

My dear Sir | Yours very sincerely | Ch. Darwin