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Show 288 TIERRA DEL FUEGO. June, 1831. subsequently to the interior ones, so as to protect a coast hitherto exposed, or that overwhelming deluges had swept down the valleys, and in some manner produced, in one day, the effects of attrition which on ordinary occasions require the prolonged action of centuries. If we could at the present day submerge the greater part of Tierra del Fuego, or leave unelevated that which we know has recently been gained, an island with a few small outliers would be formed, similar to Georgia, and situated in exactly the same latitude. Have we in such case the slightest right to deny the probability that the snow-line would descend nearly to the water's edge, and that every valley would be "terminated by a wall of ice," and that "in winter masses would be broken off and dispersed over the sea ?"-all of which circumstances are now happening in Georgia. The currents, which always set from the westward towards the east, would drift these floating masses through the channels towards the eastern side. And as we know that icebergs .at the present day, in both hemispheres, occasionally transport fragments of rock, so we cannot deny that those of Tierra del Fuego might formerly have done so. When the land was elevated, the fragments of rock would be found deposited on the. eastern side of the continent, in bands representing the ancient channels. Whether or not the hypothesis of their transport be true, such is the position of the erratic blocks in Tierra del Fuego. With respect to the general theory of the transport by great fragments of ice, especially of such as are angular, I may add a few remarks. Humboldt having observed that n.one occurred over the vast intertropical plains of the eastern s1de of South America, believed that they were entirely absent from the whole continent. As far as I am able to discover from the works of travellers, and from what I have myself seen, the remark holds good in the countries on both sides of the Cordillera as far south as central Chile. Azara has particularly stated such to be the case in Chaco. With respect to the tributaries of the Amazons, nothing can more June, 1834. ERRATIC BLOCKS. 289 strongly prove it than La Condamine's* story. He says, "Below Borja even for four or five hundred leagues, a stone, even a single flint, is as great a rarity as a diamond would be. The savages of those countries don't know what a stone is, and have not even a notion of it. It is diversion enough to see some of them when they come to Borja, and first meet with stones, express their admiration at them with signs, and be eager to pick them up, loading themselves therewith as with a valuable merchandise." It is therefore a remarkable circumstance that as soon as we reach the colder latitudes in the southern hemisphere (from 41° to Cape Horn), the same phenomenon occurs, almost on as grand a scale and with similar limits, as in the northern parts both of the Old and New World. Neither in the southern nor in the northern hemisphere do the fragments, coming from the polar regions, or from other mountain groups, arrive within a considerable distance of the lines of the tropics. We must couple the absence of erratic blocks along that part of the Andes which is situated under a warmer climate, with the similar non-occurrence, as I am informed by Professor Royle, in Northern India round the flanks of the Himmalaya ;-those loftiest pinnacles on the face of the globe. With regard to Southern Africa, from lat. 35° to the tropic, Dr. Andrew Smith, who has visited as a naturalist so large a portion of the interior, assures me he has never seen any thing of the kind. Nor do I recollect meeting with any mention of them, in the works of the numerous travellers in the equatorial regions of the same continent. The same remark certainly holds good with Australia in the parallel of Sydney, but perhaps is more doubtful with respect to Van Diemen's Land.t To my mind these negative factst have • La Condamine's Voyage (English translation), p. 24. t I will here put together all the (apparent?) exceptions which I have met with to the supposed law that erratic blocks are absent in the intertropical regions of the world. First, in the Bulletin de la Societe Geologique, 1837, p. 234, there is an account of some erratic blocks n:ar Macao (lat. 22° N.); but as it is distinctly stated they are all of gramte, VOL. HI. U |