TALUS SLOPES. 269 



about Bonito, Garanhuns and Aguas Bellas, in the state of Pernam- 

 buco, and wherever granites or gneisses form mountains throughout the 

 •country. 



Some of the best talus slopes of this type at Rio de Janeiro are those 

 at the south base of the Corcovado and at the east base of the Gavea. 



In a few instances I have seen talus slopes in Brazil which more nearly 

 resemble those of cold climates than the ones just mentioned. There 

 are such slopes along the north base of the Serra d'ltabaiana, in the 

 state of Sergipe. The underlying rocks there are gneisses, while the im- 

 mediately overlying ones are hard, resisting quartzites, and as the beds 

 dip away toward the southeast at an angle of about 30° the fragments 

 of the decaying outcrop roll down the northwest slope. These rocks 

 resist weathering influences sufficiently well to form a true talus slope. 



There are talus slopes also at the bases of the phonolite peaks of the 

 island of Fernando de Noronha.* The geology of that island, however, 

 is different from that of most of the Brazilian mainland, while the en- 

 croachment of the sea tends to remove the soil and to keep the surface 

 rocks fresh. 



EX FOLIA TION. 



In general. — Where massive crj^stalline rocks are openly exposed they 

 decay at the immediate surface and they also break up by a process of 

 exfoliation. These processes produce characteristic forms, both for the 

 rock fragments, large and small, and for the peaks and exposed parts of 

 hills and mountains. These weathered surfaces are of two types, pro- 

 duced by — 



a. Exfoliation in concentric layers, leaving rounded surfaces and in 

 some cases sharp peaks. This process affects mountain masses as well 

 as boulders. 



b. Disintegration in vertical trenches of rounded sides, leaving the rock 

 surface with a corrugated or fluted appearance. 



Exfoliation of peaks. — The topographic forms produced by the exfolia- 

 tion of openly exposed large masses of granite and gneiss are as a rule 

 so characteristic that they often afford valuable suggestions to the geol- 

 ogist, even at long distances, regarding the nature of the rocks. They 

 are especially serviceable in reconnoissance work along lines of contact 

 between crystalline and sedimentary rocks through the mountainous and 

 thickly wooded regions. 



Exfoliated peaks and bosses are so abundant about Rio as to give char- 

 acter to the scenery on every side, and such forms extend up and down 

 the coast and inland almost everywhere that granite and gneiss exist. 



* Geology Fernando de Noronha. Amer. Jour. Sci., vol. xxxvii, 1889, pp. 14.5-161. 



