These excellent images depicting "ferruginous variation" are from the publication of a paper, “On the Disposition of Iron in Variegated Strata”, delivered by the botanist and geologist George Maw to the Geological Society on April 22nd 1868. Maw begins: "Of those secondary changes which have modified the original chemical and physical constitution of rocks none seem to have more largely affected their aspect than the recombinations and rearrangements of iron". The piece is heavily illustrated throughout but it is the wonderful series of coloured plates which stand out: their diverse combinations of colour and form, at times, as though microscopic magnifications from a biology textbook, and at others, as if some kind of geological precursor to the 50s experiments of Abstract Expressionism.