It has occurred to me, that you possibly might like to see the specimen of obsidian & some curious, most finely, laminated obsidians & trachytic rocks, which I collected at Ascension island. You would not of course, I presume, think it worth the expence of carriage to have the specimens sent to Edinburgh; but shd you at any time come to London, I should be proud, if you so like, to send them for your inspection. If the subject of the lamination of volcanic rocks should interest you, I would venture to ask you to refer to p. 65-72 of my small volume, Geolog: Obser: on Volcanic Isl. which would be in the Public Library. I there throw out the idea, that the structure in question may, perhaps, be explained by your views on the zoned structure of glacier-ice: the layers of less tension, I may add, being in the case of the Ascension Obsidian-rocks, rendered apparent chiefly by the crystalline & concretionary action superinduced in them, instead of as in your ice, by the congelation of water.
Charles Darwin in his correspondence to James Davis Forbes dated 11 October (1844)