discussing Horner's paper on the geology of the Nile basin, the first letter suggesting a few cuts and politely querying his reference to "baked sandstone" ("...I have seen great areas coated with from 1 or 2 to (I think) 10 or 12 feet of dark brown, & black & angularly vesicular rock, having a most curious vitrified appearance, but which certainly was owing to a curious modification of the oxides of iron contained in it. Certainly there had been no heat from below, & certainly the whole deposit is simply superficial, & alluvial..."), the second letter further elaborating his account of ferruginous beds and their striking appearance ("...Another [naval officer] on coast of Australia said that the whole country seemed to have been fused by a flash of Lightning..."), 12 pages, 8vo, Down, Farnborough, Kent, 18 March and 27 April 1855