EXFOLIATION OF BOULDERS. 279 



on the Serra do Mundo Novo, near Rio Pardo, about 100 miles west- 

 southwest of Ilheos, Bahia.* 



Between Rio do Peixe and Joazeiro, in Bahia, the ridges and flanks of 

 the Serra do Rio do Peixe are strewn with " gigantic isolated boulders 

 •of gneiss of peculiar forms, "f Morro do Lopes, between Bahia and 

 Joazeiro, is capped by gigantic rounded boulders.J 



At Boqueirao, on Rio Grande, in the western part of Bahia, there are 

 many huge gneiss boulders of decomposition.! 



In the interior of Sergipe and of Alagoas boulders of decomposition 

 are formed from the Paleozoic (?) mountain ranges of Itabaiana and 

 Maraba inland as far as the gneiss extends. From Pao d'Assucar, on 

 the Rio Sao Francisco, to Aguas Bellas, in the interior of Pernambuco, 

 gneiss is the prevailing rock, and the boulders of decomposition are 

 abundant and vary in diameter from a few inches to 15 feet and 

 more. 1 1 



In Pernambuco there are many examples along Rio Formozo, north- 

 west of the Tertiary sediments. They are abundant near the coast wher- 

 ever the sedimentary beds have been removed from the underlying 

 granites. In the mountain regions north and west of Palmares they are 

 abundant and very large. In the state of Parahyba do Norte I did not 

 reach the gneiss belt, but the description of the rocks by Williamson 

 leads one to believe that the geology of that state is in nowise different 

 from that of Pernambuco.^ 



The " rocks of remarkable forms " seen by Koster near Agu, in Rio 

 Grande do Norte I take to be boulders of decomposition.** 



There are gneiss boulders near the base of the Serra dos Cariris in the 

 southwestern part of Ceara and along the Serra Vermelha, in the eastern 

 part of Piauhy.ft 



The only observations I have been able to make upon the subject in 

 the valley of the Amazon were made at and above the falls of the Ara- 

 guary, northeast of Macapa. At that place the granites have broken up 

 into boulders, all of which are rounded by exfoliation. They occur at 

 Pedreira near the junction of the Rio Negro and Rio Branco.JJ A little 

 below Moura the islands and boulders in the Rio Negro are all rounded. §§ 



* Voyage au Bresil. Prince Maximilian de Wied-Neuwied. Paris, 1822, iii, pp. 61, 63. 

 iSpix u. Martius : Reise in Brasilien, ii, p, 722. 



t Theodore Fernandes Sampaio: Revista de Engenhavia, vi, 1884, p. 53. 

 § Three Thousand Miles through Brazil. J. W. Wells, London, 1886, vol. ii, p. 69. 

 II American Naturalist, Dec, 1884, p. 1189. 



Tf Geology of Parahiba and Pernambuco gold regions. E. Williamson. Trans. Manchester GeoL 

 Soc, vol. vi, pp. 113-121, 

 ** Travels in Brazil. Henry Koster. London, 1817, vol. i, p. 141. 

 ft Travels in Brazil. George Gardner. Pp. 232, 237, 238. 

 tt Journey in Brazil, p. 418. 

 ^§ Spix u. Martius: Reise in Brasilien, iii, p. 1294. 



