Darwin was horrified that his careless words were in print, but "No hay remedio" (it can't be helped). He explored the geology of the area, reaching conclusions about slate formation and the injection of granite seams as liquid which differed from the ideas of Lyell and Sedgwick. The zoologist Andrew Smith showed him formations, and later discussed the large animals living on sparse vegetation, showing that a lack of luxuriant vegetation did not explain the extinction of the giant creatures in South America. Around 15 June Darwin and FitzRoy visited the noted astronomer Sir John Herschel. In his diary Darwin called this "the most memorable event which, for a long period, I have had the good fortune to enjoy." His zeal for science had been stirred at Cambridge by reading Herschel's book on philosophy of science, which had guided his theorising during the voyage. Their discussion is not recorded, but a few months earlier, on 20 February 1836, Herschel had written to Lyell praising his "Principles of Geology" as a work which would bring "a complete revolution in [its] subject, by altering entirely the point of view in which it must thenceforward be contemplated." and opening a way for bold speculation on "that mystery of mysteries, the replacement of extinct species by others." Herschel himself thought catastrophic extinction and renewal "an inadequate conception of the Creator", and by analogy with other intermediate causes "the origination of fresh species, could it ever come under our cognizance, would be found to be a natural in contradistinction to a miraculous process". In Cape Town missionaries were being accused of causing racial tension and profiteering, and after "Beagle" set to sea on 18 June FitzRoy wrote an open letter to the evangelical "South African Christian Recorder" on the "Moral State of Tahiti" incorporating extracts from both his and Darwin's diaries to defend the reputation of missionaries. This was given to a passing ship which took it to Cape Town to become FitzRoy's (and Darwin's) first published work. On 8 July they stopped at St. Helena for six days. Darwin took lodgings near Napoleon's tomb, and when writing to Henslow asking to be proposed for the Geological Society, mentioned his suspicions "that differently from most Volcanic Islds. its structure is rather complicated. It seems strange, that this little centre of a distinct creation should, as is asserted, bear marks of recent elevation." With a guide he wandered over the island, noting its complex sloping strata showing fault lines, interlaced with volcanic dykes.