In 1872 Agassiz went through the Straits of Magellan in charge of the natural history work of the Hassler Expedition. On that voyage he touched at Montevideo and at many points south of that place, through the straits, and along the west coast. The letters written by him on this trip suggest very strongly, if they do not conclusively show, that he had at this time already abandoned the idea that Brazil had been glaciated. Speaking of certain boulders seen by him on the Cerro at Montevideo, Mrs. Agassiz observes that "As these were the most northern erratics and glaciated surfaces reported in the southern hemisphere," etc. From this it appears that he no longer regarded the Brazilian boulders as erratics. After Agassiz had examined the glacial phenomena of the Straits of Magellan and of the southern part of the continent, he sent a report to the Superintendent of the U. S. Coast Survey, dated at Concepcion Bay, June 1, 1872.2 This article also bears evidence that he no longer regarded Brazil as having been glaciated. In one place he says,3 "I am prepared to maintain that the whole southern extremity of the American continent has been uniformly moulded by a continuous sheet of ice." The italics are mine. In the next paragraph he says, "The first unquestionable roches moutonnées I saw were upon the nearest coast opposite Cape Froward." Again he says (p. 271): "The equatorial limit of this ice sheet both in the northern and the southern hemisphere is part of the problem upon which we have thus far fewest facts in our possession. In South America I have now traced the facts from the southernmost point of the continent uninterruptedly to 37° S. latitude on the Atlantic as well as the Pacific coast." Again 'Louis Agassiz, his Life and Correspondence, Boston, 1886, II., 712. Rep. U. S. Coast and Geodetic Survey for 1872, 215. Nature, 1872, VI., 69. Evidently Burmeister does not regard the boulders cited as glacial, for he uses the expression, "phénomènes de glaciers chez nous, et dont nous n'avons nulle part la preuve." République Argentine, II., 214, also 392, 393. The same blocks are described by Darwin in his Geolog. ical Observations, 432. He does not seem to regard them as erratics. 2 Published in the New York Tribune of June 26, 1872, and reproduced in Nature 1872, VI., 216, 229 and 260. 3 Nature, 1872, VI., 230. (p. 272) he speaks of having traced the palpable evidence of glaciation "from Montevideo on the Atlantic to Talcahuano on the Pacific coast." Speaking of evidence at Concepcion Bay he says also (p. 272) "Think of it! A characteristic surface indicating glacial action in latitude 37° S. at the level of the sea!" These quotations show as plainly as anything short of a positive statement can that Agassiz in 1872 no longer considered as trustworthy what he had formerly regarded as the evidences of glaciation in Brazil. For if he still believed in a glacier under the equator itself, why should he tell us with exclamation points to think of a glacier thirty-seven degrees nearer the pole ? BASIS OF THE THEORY. I should be glad to leave the matter with these statements of the changes of views on the part of both advocates of the glaciation of Brazil, but persons who have theories based to a greater or less extent on the glaciation of the tropics are very reluctant to believe, in the face of the many positive statements of both Agassiz and Hartt, and of the apparently trustworthy evidence adduced by them, that the first impressions of those excellent observers, both of whom were thoroughly familiar with glacial phenomena in the north, were altogether wrong. It is not possible, neither is it necessary, to take up here the individual cases spoken of by Agassiz and Hartt as evidence of glacial action. Very nearly all the materials referred by them to the drift fall under two principal heads: First, the so-called erratic boulders, often imbedded in what was considered boulder-clay. Second, transported, water-worn materials. ORIGIN OF THE BOULDERS. The boulders believed to be erratics are not erratics in the sense implied, though they are not always in place. The first and most common are boulders of decomposition, either rounded or subangular, left by the decay of granite or gneiss. Sometimes they are imbedded in residuary, and consequently unstrati fied clays, formed by the decomposition in place of the surrounding rock. And everyone has heard of the great depth to which rocks are decomposed in Brazil. The true origin of these boulders and the accompanying clays is often more or less obscured by the "creep" of the materials, or, in hilly districts, by land-slides, great or small, that throw the whole mass into a confusion closely resembling that so common in the true glacial boulder-clays. In this connection too much stress can scarely be placed upon the matter of land-slides; they are very common in the hilly portions of Brazil, and, aside from profound striations and faceting, produce phenomena that, on a small scale, resemble glacial till in a very striking manner. The fact that the boulders are of various sizes, sometimes from ten to twenty feet in diameter, and have mingled with them quartz fragments derived from the veins that traverse the crystalline rocks from which they are derived, adds to the resemblance of these materials to certain glacial products. Such boulders, however, are by no means confined to the vicinity of Rio de Janeiro, but are common throughout Brazil wherever there are granites or gneisses. They have been seen by the writer in the Amazon valley (Araguary River) in the interior of Pernambuco, Parahyba do Norte, Alagoas, Sergipe, Bahia, Rio de Janeiro, Minas Geraes, São Paulo, Paraná, and Matto Grosso. 2 The positions in which such boulders are often found are worthy of note, though one who felt disposed to regard them as transported blocks would probably not consider their positions as inconsistent with the glacial theory of their orgin. They are abundant about the bases of granite hills and mountains where they have been formed by the exfoliation of the great blocks and slabs produced by the secular decay of the hills and mountains. There are hundreds of rude boulders at the southeast base 'DARWIN Geological Observations, 427; LIAIS: Climats, Géologie, etc., 2; PISSIS: Men. Hist. Inst. de France, X., 538; DERBY: Amer. Jour. Sci., 3d Ser., XXVII., 138; MILLS: Amer. Geologist, III., 351. In the American Naturalist, 1884, XVIII., 1189, I have given a sketch of some boulders found in the state of Pernambuco; see also p. 1187 of that vol. |