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Show 614 ADDENDA. zones, tbat solid glaciers descend to the sea in low latitudes ; I might have added that' it is in this same hemisphere, that the icebergs, which have been formed in the Polar Regions, are drifted furthest from their birthplace. Horsburgh ( Pltilosoplt. Transact., 1830) describes several great icebergs seen by a ship, in her passage to India, in 35° 50' S. : that is, far to the northward of the latitude, where tree-ferns, arborescent grasses, parasitical orchideous plants, and even palm-trees grow; and within sixty miles of the land, where the rhinoceros, elephant, hippopotamus, lion,and hyena, are very numerous. Page 289. Until lately I was not aware that there were sufficient data to speak with some precision of the southern limits of erratic blocks in the northern half of the New World. In Canada, and in the northern parts of the United States, innumerable great scattered fragments of rocks have been described by Bayfield, Bigsly, Hitchcock, and others. In parts of Massachusetts, according to Professor Hitchcock (Report on tlte Geologg of), boulders seem to cover the whole face of the country. Further southward we hear from Mr. Rogers (Report to Brit. Assoc., vol. iii.) that boulders are common over the great valley which crosses Pennsylvania, Maryland, and Virginia (lat. 36° 30' to 42°): and likewise in the states of Ohio, Kentucky, and Indiana, which are in nearly the same latitude. Mr. Rogers having described some blocks of sandstone at Washington and on the Susquehanna, which must have come from some distance northward, adds that " Drake in his picture of Cincinnati (39° 10') mentions large masses of granite in that part of the Obio, resting on the ordinary finer diluvium. The nearest granite to the north is at least one hundred leagues distant; while no primary rock occurs south or east, within even a much greater limit." He_then proceeds, " We are reminded here of the great detached blocks, which strew the plains of northern Europe, and the explanation suggested, that they have been carried there upon floating ice;" and concludes with the important remark, that Mr. Conrad, who has explored the state of Alabama (30° to 350) was never once able to perceive a boulder upon its surface. It would hence appear that 36° 30' is the southern limit of the dispersion of erratic blocks in the United States; and these are spoken of, as having come from the north. Therefore, there is no occasion to suppose that the ice, in which by the theory they are believed to have been embedded, was formed in so low a latitude as that here mentioned ; and at present, in the southern hemisphere, icebergs are drifted to latitudes, though not formed in them, nearer the tropic than 36° 30'. In Europe I cannot hear of erratic blocks having been found fnrthet· south than the southern flanks of the Alps, in lat. 45°; and Hum- ADDENDA. 615 boldt has said (see Cuvie1·'s Theory of the Ea1'th, translated by Professor Jameson, p. 346) that they do not occur in Lombardy. I may here remark, that care should be taken to separate the phenomenon of great angular blocks, from that of rounded ones, although of considerable size; for torrents, ·and more especially the waves of the sea, duriug its slow oscillation of level, are agents sufficiently powerful to produce great effects. The lowest latitude in South America, in which I found large angular fragments, which must have been transported by ice there formed, or by some unknown means, was in latitude 41°. But as I did not examine the country immediately north of it, I am not prepared to say that this is their extreme limit; but between latitude 27° and 33°, I found no appearance, on either side of the Cordillera, which indicated a power of transportation of the kind required to remove boulders from a distance. Thus, we find that the limit of their dispersion in the two Americas is nearly the same; although thPy approach the warmer zones rather more closely in the northern than in the southern division of the continent, and in both, probably, more so than in Europe. In the note, in which I have considered the apparent exceptions to the law, that erratic boulders are not found in the intertropical regions, I have said that the internal evidence of the Macao case led me to doubt its reality, and I now find it is distinctly stated by M. Chevalier that the rounded blocks result from the secular disintegration of the fundamental rock (L'Institut, 1838, p. !51-Analysis of t!te Voyage of the Bonite). I may here add, that M. Puillon Boblaye, in his description of Bone and Constantine on the northem coast of Africa ( L'Institut, 1838, p. 248, says," Je n'ai rien vu que put indiquer le phenomcne des blocs erratiqnes." My statement that erratic boulders are not found in Australia, is fully borne out by information communicated to me by Major Mitchell, who, in his repeated expeditions, has traversed so much of the south-east division of that continent. With the several facts given here and in the Journal (p. 289), I can scarcely doubt that the Jaw of the distribution of erratic blocks is finally determined; and it is needless to specify the great, not to say conclusive, importance of this law on the theory of the means of their transportation, - a problem which has so long perplexed geologists. Page 294. In my discussion on the climate of the southern hemisphere, I have shown that a low altitude of the line of perpetual snow, and consequently the descent of glaciers to the level of the sea in latitudes relatively low to what occurs in the northern hemisphere, and likewise the perpetual congelation of the soil a little beneath the surface in countries without the |