Page:The Journal of geology (IA journalofgeology11893univers).pdf/339

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.^[1] 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,^[2] Parahyba do Norte, Alagôas, Sergipe, Bahia, Rio de Janeiro, Minas Geraes, São Paulo, Paraná, and Matto Grosso.

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