The indelible impression made on Darwin’s mind by this first visit to a volcanic island, is borne witness to by a remarkable passage in the “Autobiography” written by him in 1876. “The geology of St. Jago is very striking, yet simple; a stream of lava formerly flowed over the bed of the sea, formed of triturated recent shells and corals, which it has baked into a hard white rock. Since then the whole island has been upheaved. But the line of white rock revealed to me a new and important fact, namely that there had been afterwards subsidence round the craters which had since been in action, and had poured forth lava. It then first dawned on me that I might perhaps write a book on the geology of the various countries visited, and this made me thrill with delight. That was a memorable hour to me, and how distinctly I can call to mind the low cliff of lava beneath which I rested, with the sun glaring hot, a few strange desert plants growing near and with living corals in the tidal pools at my feet.” Only five years before, when listening to poor Professor Jameson’s lectures on the effete Wernerianism, which at that time did duty for geological teaching, Darwin had found them “incredibly dull,” and he declared that “the sole effect they produced on me was a determination never so long as I lived to read a book on Geology, or in any way to study the science.” What a contrast we find in the expressions which he makes use of in referring to Geological Science, in his letters written home from the _Beagle_! After alluding to the delight of collecting and studying marine animals, he exclaims, “But Geology carries the day!” Writing to Henslow he says, “I am quite charmed with Geology, but, like the wise animal between two bundles of hay, I do not know which to like best; the old crystalline group of rocks, or the softer and more fossiliferous beds.” And just as the long voyage is about to come to a close he again writes, “I find in Geology a never-failing interest; as it has been remarked, it creates the same grand ideas respecting this world which Astronomy does for the Universe.” In this passage Darwin doubtless refers to a remark of Sir John Herschel’s in his admirable “Preliminary Discourse on the Study of Natural Philosophy,”—a book which exercised a most remarkable and beneficial influence on the mind of the young naturalist.