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

574 GEOLOGICAL REMARKS. and nature of the ranges, and the direction of the shores : the hills are irregularly heaped together ; the sounds are intricate and tortuous in their course, and the shores are formed by deep sinuosities and prominently projecting headlands: the channels, also, are studded with innumerable islands and rocks extremely dangerous for navigation. In this portion the rock is, for the most part, granite and greenstone. Near the centre of the Strait, the rock being clay-slate, the mountains are higher, and more precipitous and rugged in their outline ; and consequently not easily to be ascended. They are in general three thousand feet, but some are found to be four thou- sand feet, in height ; and one. Mount Sarmiento, is upwards of six thousand feet high, and is covered throughout the year with snow. The line of perpetual snow in the Strait seems to be about three thousand five hundred feet above the sea : the mountains, whose height does not exceed three thousand, are, during the summer, frequently free from any, excepting in holes, where a large quantity is accumulated by drifting, and protected from the sun. The Strait here is quite free from islands, and it is a remark- able fact, that where the greenstone formation terminates, there the islands cease to appear. The slate formation continues as far as Freshwater Bay, where the stratified rocks leave the coast and extend in a north-west direction. The soil then becomes apparently a mixture of decom- posed slate and clay ; the slate gradually disappearing on approach- ing to Cape Negro, where the rock partakes of the character of the east coast. Here again we observe, along with the change of geological character, the re-appearance of islands, the soil of which is clayey, but with masses of granite, hornblende rock and clay slate protruding in many places through the superfi- cial soil, which, although it yields a poor grass, is entirely desti- tute of trees. In that portion of the Strait to the eastward of Cape Negro the hills are remarkable for the regularity and parallelism of their direction, and their general resemblance to each other. On the north shore, near Cape Gregory, a range of high land commences suddenly, with rather a precipitous ascent, and extends for forty iTdiles to the north-east, where it terminates in detached rocky hills. The south-western end of the range is a ridge of flat- topped