EXFOLIATION. 277 



and another of the table-topped Gavea. Some great domes of soUd rock were 

 visible. ..." * 



In the gneiss region of the Pernambuco these rounded forms are com- 

 mon in the mountains west of the city, about Bonito and Aguas Bellas. 

 In the northern part of Brazil, however, these mountains are not so lofty 

 as they are about Rio de Janeiro and south of that city along the coast. 

 In Ceara the rounded and exfoliated hills are common in the gneiss 

 regions also. At and about Quixada such forms are so common that 

 scores of them may be seen in the immediate vicinity. There is a good 

 example at the end of the great Quixada dam.f 



Near Sobral is one of these isolated granite peaks, popularly supposed 

 to be a volcano.! 



Mr Darwin § suspects " that the boldly conical mountains of gneiss- 

 granite near Rio de Janeiro, in which the constituent minerals are ar- 

 ranged in parallel planes, are of intrusive origin." This arrangement of 

 the minerals is a feature of Brazilian gneiss which is not confined to the 

 peaks alone. 



Exfoliation of houlders. — In Brazil boulders of decomposition are char- 

 acteristic of, and so far as I am aware are confined to, the regions of 

 crystalline rocks. They are common about Rio de Janeiro, where they 

 are sometimes as much as 50 feet in diameter. The noted boulders below 

 the hotel at Tijuca, cited by Agassiz and Hartt in evidence of the glacia- 

 tion of Brazil, are boulders of decomposition, some of which have rolled 

 down from the mountains above; || other large ones lie upon the south 

 side of the Tijuca peak, in the forest. Excellent examples are abundant 

 on the islands in the bay of Rio de Janeiro. 



Plate 13 shows some of them on the beach on the island of Paqueta 

 in the bay. There are many other small islands in the bay, especially 

 about the Ilha do Governador, on which they are abundant, while at 

 many places heaps of them appear above water where the soil and softer 

 materials have been removed.^ There are similar boulders on the sum- 

 mit of the Sugar Loaf and of the Gavea ; there are many of them about 

 the south base of the Sugar Loaf There are several very fine ones at 

 the foot of the hills on the east side of Lagoa Rodrigo de Freitas, and 

 others at the south base of the Gavea. Eschwege records many about 



♦Around and about South America. Frank Vincent. 5th ed., New York, 1895, p. 313. 



t The state of Ceara. Jos6 Freire Fontenelle. Chicago, 1893, p. 32. 



X Trabalho da Commissao Scientifiea de Explora^ao. I, Tniroducgao, Rio de Janeiro, 1862. Re- 

 latorio da Sec§ao geologica. G. S. da Capenena, cxxxix. 



§ Geological Observations, p. 468. 



II Agassiz: Journey in Brazil, p. 86. Hartt: Geology and Phys. Geog. of Brazil, pp. 28-30. Bran- 

 ner : Jour, of Geol., vol. i, 1893, p. 764. 



^ See also Reise nach Brasilien. H. Burmeister. Berlin, 1853, iii, p. 112. See also South Ameri- 

 can Sketches. T. W. Hinchliff. London, 1863, p. 224. 



