Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.1): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

576 VEGETATION. From the regularity of the direction of the strata in the slate districts the vallies are very extensive, and, being bounded on either side by precipitous mountains much intersected by deep ravines, receive large streams of water, which, uniting together in their course to the sea, form no inconsiderable rivers. During the winter months these rivers become swollen and overflow their banks, and deposit a quantity of alluvium, which, blending with the fallen leaves and other putrescent substances, produces a good superficial soil, in which trees grow to a large size, and the shrubs and smaller plants become particularly luxuriant and productive. At Port Famine, and in its neighbourhood, the evergreen beech {Fagus betuloides) grows in the greatest abundance, and reaches a very large size. Trees of this species, of three feet in diameter, are abundant ; of four feet, there are many ; and there is one tree (perhaps the very same noticed by Commodore Byron*), which measures seven feet in diameter for seventeen feet above the roots, and then divides into three large branches, each of which is three feet through. This venerable tree seemed to be sound, but from our experience of several others that were cut down, might be expected to prove rotten in the centre. This tendency to decaying in the heart may be attributed to the cold- ness of the schistose sub- soil upon which the trees are rooted, as well as to the perpetual moisture of the climate above alluded to. The slate formation ceases at Port St. IMary, but there is no decided change in the vegetation until we come to Cape Negro, where the clay commences ; and from thence onwards there is not a tree to be found. The nature of the soil is not favourable to plants which take a deep root, and, therefore, only shrubs and grasses are found : the former are thinly scattered over the ex- tensive plains which characterise this country ; but the grasses are abundant, and although of a harsh and dry appearance, must be nourishing, for they form the chosen food of numerous and large herds of guanacoes. Besides the evergreen beech above-mentioned, there are but few other trees in the Strait that can be considered as timber trees. Such an appellation only belongs to two other species of beech and the Winter's bark. The last, which is also an evergreen, is to be found mixed with the first, in all parts of the Strait ; so that * Hawkesworth, Voyages, i. 38.