Professor Henslow, the botanist, through whom Darwin had been offered the opportunity to accompany the expedition, had presented his pupil with the first volume of Lyell's _Principles of Geology_. (Perhaps, after Lyell, the most potent influence on Darwin's mind at this time was that of Humboldt and other renowned travelers, whose works he read with avidity.) At the Cape Verde Islands he made some interesting observations of a white calcareous stratum which ran for miles along the coast at a height of about forty-five feet above the water. It rested on volcanic rocks and was itself covered with basalt, that is, lava which had crystallized under the sea. It was evident that subsequently to the formation of the basalt that portion of the coast containing the white stratum had been elevated. The shells in the stratum were recent, that is, corresponded to those still to be found on the neighboring coast. It occurred to Darwin that the voyage might afford material for a book on geology. Later in the voyage, having read portions of his _Journal_ to Captain Fitzroy, Darwin was encouraged to believe that this also might prove worthy of publication. Darwin's account of his adventures and manifold observations is so informal, so rich in detail, as not to admit of summary. His eye took in the most diverse phenomena, the color of the sea or of rivers, clouds of butterflies and of locusts, the cacique with his little boy clinging to the side of a horse in headlong flight, the great earthquake on the coast of Chile, the endless variety of plant and animal life, the superstition of savage and _padre_, the charms of Tahiti, the unconscious humor of his mountain guides for whom at an altitude of eleven thousand feet "the cursed pot (which was a new one) did not choose to boil potatoes"--all found response in Darwin's open mind; everything was grist to his mill. Any selection from the richness of the original is almost sure to show a tendency not obvious in the _Journal_. On the other hand, it is just such multiplicity of phenomena as the _Journal_ mirrors that impels every orderly mind to seek for causes, for explanation. The human intellect cannot rest till law gives form to the wild chaos of fact.