<P> Six days later they made their first landing at Porto Praya on the volcanic island of St. Jago in the Cape Verde Islands . It is here that Darwin's description in his published Journal begins . His initial impression was of a desolate and sterile volcanic island, but after visiting the town he came to a deep valley where he "first saw the glory of tropical vegetation" and had "a glorious day", finding overwhelming novelty in the sights and sounds . FitzRoy set up tents and an observatory on Quail Island to determine the exact position of the islands, while Darwin collected numerous sea animals, delighting in vivid tropical corals in tidal pools, and investigating the geology of Quail Island . Though Daubeny's book in the Beagle's library described the volcanic geology of the Canary Islands, it said that the structure of the Cape Verde Islands was "too imperfectly known". Darwin saw Quail Island as his key to understanding the structure of St. Jago, and made careful studies of its stratigraphy in the way he had learnt from Adam Sedgwick . He collected specimens and described a white layer of hard white rock formed from crushed coral and seashells lying between layers of black volcanic rocks, and noted a similar white layer running horizontally in the black cliffs of St. Jago at 40 feet (12 m) above sea level . The seashells were, as far as he could tell, "the same as those of present day". He speculated that in geologically recent times a lava flow had covered this shell sand on the sea bed, and then the strata had slowly risen to their present level . Charles Lyell's Principles of Geology presented a thesis of gradual rising and falling of the Earth's crust illustrated by the changing levels of the Temple of Serapis . Darwin implicitly supported Lyell by remarking that "Dr. Daubeny when mentioning the present state of the temple of Serapis . doubts the possibility of a surface of country being raised without cracking buildings on it .--I feel sure at St Jago in some places a town might have been raised without injuring a house ." In a letter to Henslow he wrote that "The geology was preeminently interesting & I believe quite new: there are some facts on a large scale of upraised coast...that would interest Mr. Lyell . While still on the island, Darwin was inspired to think of writing a book on geology, and later wrote of "seeing a thing never seen by Lyell, one yet saw it partially through his eyes". </P> <P> Customarily the ship's surgeon took the position of naturalist, and the Beagle's surgeon Robert McCormick sought fame and fortune as an explorer . When they first met at the start of the voyage, Darwin had commented that "My friend (McCormick) is an ass, but we jog on very amicably". They walked into the countryside of St. Jago together, and Darwin, influenced by Lyell, found the surgeon's approach old - fashioned . They found a remarkable baobab tree, which FitzRoy measured and sketched . Darwin went on subsequent "riding expeditions" with Benjamin Bynoe and Rowlett to visit Ribeira Grande and St Domingo . FitzRoy extended their stay to 23 days, to complete his measurements of magnetism . Darwin subsequently wrote to Henslow that his collecting included "several specimens of an Octopus, which possessed a most marvellous power of changing its colours; equalling any chamaelion, & evidently accommodating the changes to the colour of the ground which it passed over.--yellowish green, dark brown & red were the prevailing colours: this fact appears to be new, as far as I can find out ." Henslow replied that "The fact is not new, but any fresh observations will be highly important ." </P> <P> McCormick increasingly resented the favours FitzRoy gave to assist Darwin with collecting . On 16 February, FitzRoy landed a small party including himself and Darwin on St. Paul's rocks, finding the seabirds so tame that they could be killed easily, while an exasperated McCormick was left circling the islets in a second small boat . That evening novices were greeted by a pseudo-Neptune, and in the morning they crossed the equator with the traditional line - crossing ceremony . </P> <P> Darwin had a special position as guest and social equal of the captain, so junior officers called him "sir" until the captain dubbed Darwin Philos for "ship's philosopher", and this became his suitably respectful nickname . </P>

What was the purpose of the voyage of the hms beagle