Lyell had been very early convinced of the supreme importance of travel to the geologist. In a letter to his friend Murchison he said:--'We must preach up travelling, as Demosthenes did "delivery" as the first, second and third requisites for a modern geologist, in the present adolescent state of the science[44].' And Professor Bonney has estimated that so far did he himself practise what he preached, that no less than one fourth of the period of his active life was spent in travel[45]. The joint excursion of Lyell and Murchison to the Auvergne was destined to have great influence on the minds of these pioneers in geological research; both became satisfied from their studies that, with respect to the excavation of the valleys of the country, Scrope's conclusions were irresistible; and in a joint memoir this position was stoutly maintained by them. It is interesting to notice the impression made by these two great geologists on one another during this joint expedition. Murchison wrote that he had seen in Lyell 'the most scrupulous and minute fidelity of observation combined with close application in the closet and ceaseless exertion in the field[46].' But I recollect that Lyell once told me how difficult Murchison found it to restrain himself from impatience, when his companion's attention was drawn aside by his entomological ardour. In an early letter, indeed, we find that Murchison often expressed a wish that Lyell's sisters had been with them to attend to the insect-collecting and thus leave Lyell free for geological work[47]. On the other hand, Lyell informed me that Murchison had rendered him a great service in showing how much a geologist could accomplish by taking advantage of riding on horseback, and he declared in his letters that he 'never had a better man to work with than Murchison'; nevertheless he ridiculed his 'keep-moving-go-it-if-it-kills-you' system as--quoting from the elder Matthews--he called it[48]. On parting from Murchison and his wife, after the Auvergne tour, Lyell proceeded to Italy and for more than a year he was busy studying the Tertiary deposits of Lombardy, the Roman states, Naples and Sicily, and conferring with the Italian geologists and conchologists. Thus it came about that he was not free to resume the task of seeing the _Principles_ through the press till February 1829.