SUPPOSED GLACIATION OF BRAZIL. 761 



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, resem- 

 ble 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 mate- 

 rials to certain glacial products. Such boulders, however, are by 

 no means confined to the vicinity of Rio de Janeiro, but are com- 

 mon throughout Brazil wherever there are granites or gneisses. 

 They have been seen by the writer in the Amazon valley (Ara- 

 guary River) in the interior of Pernambuco, 2 Parahyba do Norte, 

 Alagoas, Sergipe, Bahia, Rio de Janeiro, Minas Geraes, Sao Paulo, 

 Parana, 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 moun- 

 tains. There are hundreds of rude boulders at the southeast base 



1 Darwin: Geological Observations, 427; Liais : Climats, Geologie, etc., 2; 

 Pissis : Men. Hist. Inst, de France, X., 538 ; Derby : Amer. Jour. Sci., 3d Ser., 

 XXVII., 138; Mills: Amer. Geologist, III., 351. 



2 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. 



