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Show 266 'l'IERU.A DEL FUEGO. June, 1834. long on the top of the mountain. Our descent was not quite so laborious as our ascent; for the weight of the body forced a passage, 'and all the slips and falls were in the right direction. Captain King has given a sketch of the geology of Tierra del Fuego, to which I have little to add. A great formation of clay-slate, rarely containing organic remains, but sometimes presenting casts of a kind of ammonite, is fronted on the east side by plains belonging probably to two tertiary epochs. On the west coast, a prolongation of the grand crevice of the Andes, from which so much heat has escaped from the interior of the globe, has metamorphosed the slate. There is, however, a double line, the structure of which I do not quite comprehend. The interior one consists of granite and mica slate; the exterior one (perhaps more modern), of greenstone, porphyritic and other curious trappean rocks. Almost every one at first thinks that this country owes its grand name of "the Land of Fire," to the number of its volcanoes. Such, however, is not the case : I did not see even a pebble of any volcanic rock, except in Wollaston Island, where some rounded masses of scorire were embedded in a conglomerate of no modern date. In a geological point of view this circumstance allows us to consider the grand linear train of ancient and modern volcanoes . . ' whiCh fall on parallel fissures m the Andes, as extending from lat. 55° 40' south to 60° north, a distance little less than seven thousand geographical miles. Perhaps the most curious feature in the geology of this country, is the extent to which the land is intersected by arm~ of the sea. These channels, as Captain King remarks, are Irregular and dotted with islands, where the granitic and trappean rocks occur, but in the clay-slate formation are so straight, that in one instance " a parallel ruler placed on the map upon the projecting points of the south shore, extended across, also touched the headlands of the opposite coast." I have heard Captain FitzRoy remark, that on entering any of these channels from the outer coast, it is always ne- June, 1834. GEOLOGY. 267 cessary to look out directly for anchorage ; for further inland the depth soon becomes extremely great. Captain Cook, in entering Christmas Sound, had first 3 7 fathom, then 40, 60, and, immediately afterwards, no soundings with 170. This structure of the bottom, I presume, must arise from the sediment deposited near the mouths of the channels, by the opposed tides and swell·; and likewise from the enormous degradation of the coast rocks, caused by an ocean harassed by endless gales. The Strait of Magellan is extremely deep in most parts, even close to the shore. About mid-channel eastward of Cape Froward, Captain King found no bottom with 1536 feet: if, therefore, the water should be drained off, Tierra del Fuego would present a far more lofty range of mountains than it does at present. I will not here enter on any speculations regarding the causes which have produced this remarkable structure, in a district in which the latter movements at l~ast have been those of elevation. I may, however, observe, that pebbles, and great boulders of various and peculiar crystalline rocks, which have undoubtedly travelled from the south-west coast, lie scattered over the whole of the eastern part of Tierra del Fuego. One enormous block of syenite near St. Sebastian Bay was barn-shaped, and had a girth of 4 7 feet ; it projected five feet above the sand, and appeared to be deeply buried. The very nearest point to which we can look for the parent rock, is about ninety miles distant. On the shores of the Strait of Magellan, near Port Famine, numerous semi-rounded fragments of various granites and hornblendic rocks are strewed on the beach, and on the sides of the mountain, to an elevation of thirty or forty feet. Now to this point the high road from the Southern and Western shores passes directly over the great abyss of more than 1500 feet deep. Whatever may have been the means of transport, it has not been one of indiscriminate violence: for the two places, St. Sebastian Bay and Shoal Harbour, where the great fragments are most numerous, certainly existed previously to the last and |