To J. D. Forbes 13 [November 1844]1
Down near Bromley Kent
Wednesday 13th —
I had intended answering your very obliging note sooner, but have been prevented doing so. I am sorry I cannot break the specimen of zoned obsidian as it is not by own; but I should so much like you to see these specimens, that I have taken the liberty of paying the carriage & sending by steam-boat to you, directed to the University 14 fragments of rock, which even if not worth looking at, can not give you much trouble. One is apt to overrate the interest of anything one sees oneself, & I have no doubt I have done so, yet I think the series curious, as showing such perfect lamination or rather separation of minerals in parallel planes, in a pile of rock of undoubtedly volcanic origin.—
I think the Mexican obsidian, zoned with minute air-cells will interest you. Two of my specimens show the passage from obsidian into the intercallating feldspathic rocks, in which the laminar structure is chiefly rendered apparent by concretionary action; in the other specimens the laminar structure is redered apparent by crystalline action.—: one of these like gneiss appears to me interesting. All the specimens are labelled.— You can return them, whenever you like, directed to me to my Brothers House at diag “7. Park St
I have forgotten to thank you for sending me your latest contributions on glaciers:2 by a singular chance, the very day, after I had written to you, I received the Edin: New Phil. Journal & there read your remarks on the comparison of lava-streams & glaciers;3 which if I had seen sooner, I shd not have troubled you.— I have heard lately from a young German who is very full of the ice-action-marks which he discovers in Germany & Hungary4 how, I wish, someone could point out a clear line of separation, between glacier & iceberg action: on the outskirts of N. Wales, they appeared to me to blend together in the most puzzling manner. I observed one fact there, which I have always been curious to have explained; I have marked the passage in the enclosed: perhaps you can explain it—: is it due to your mobility of the ice.—5 But I have much cause to apologise for troubling you with so long a note.
Believe me, My dear Sir. Yours very faithfully C. Darwin
Mexican specimen of laminated obsidian.
Comments on Forbes’s publication comparing lava streams and glaciers. Mentions ice-action theories of a young German.
Please cite as
Darwin Correspondence Project, “Letter no. 790,” accessed on 24 July 2016, http://www.darwinproject.ac.uk/DCP-LETT-790