264 J. C. BRANNER — DECOMPOSITION OF ROCKS IN BRAZIL. 



In his itinerary in Goyaz Castelnau represents the granite just north 

 of Aldea de Carretao as much altered.^ 



Liais, who traveled extensively in Brazil, especially in the valley of the 

 Sao Francisco, says that it is no uncommon thing to find places where 

 the gneiss is changed to clay to a depth of more than 100 meters.f The 

 deep decay of the rocks has been noted in the state of Matto Grosso by 

 the Av liter, and also by Dr Severiano da Fonseca. J 



In the state of Bahia the crystalline rocks everywhere show the effects 

 of decomposition. In Sergipe and Alagoas the decay of the gneiss and 

 granite which lie inland from the Cretaceous sediments which border the 

 coast is more marked than that of the sedimentary rocks. The schists 

 along the lower Rio Sao Francisco are much affected locally, some of 

 them decaying much more rapidly than others. 



In the state of Pernambucothe rocks are mostly granites and gneisses^ 

 and these are deeph" decomposed, especially near the coast. The enor- 

 mous cuts on the Recife a Sao Francisco railway are almost all in decayed 

 granites. Similar decomposition (though not so deep cuts) and many 

 exfoliated blocks are shown at a number of places in crossing the moun- 

 tains from Palmares to Bonito and in the vicinit}^ of the latter place. 

 From Pao d'Assucar on the Rio Sao Francisco to Aguas Bellas the gran- 

 ites are sometimes deeply decayed, especiall}^ in the valleys, and boulders 

 of decomposition are common everywhere through the gneiss and granite 

 region lying inland from the schists and sedimentary rocks. 



In the state of Para the older rocks are found in place only away from 

 the river or from the main axis of the Amazon valley. They usually 

 appear at the fall line in ascending the affluents of the Amazon. At the 

 rapids first encountered in ascending the Araguary on the north side of 

 the valley the rocks are granites and are everywhere deeply decomposed 

 and weathered into exfoliated boulders. 



Agassiz often refers to the widespread decomposition of the rocks of 

 Brazil. In one place he speaks of them being '* reduced to the condition 

 of a soft paste, exhibiting all the mineralogical elements of the rocks as 

 they may have been before the}^ were decomposed, but now completely 

 disintegrated ; " § but though he makes frequent reference to the wide- 

 spread decomposition of the rocks he gives but few measurements of the 

 actual de])th to which he found it to extend. In one place he speaks of 



* Expedition dans rAm6rique duSud. IVmepartie. Itineraire et Coupe G6ologique. Planche 12. 



t Climats, G6ologie du Br6sil. E. Liais. Paris, 1872, p. 2. 



I Viagem ao redor do Brazil. Rio de Janeiro, 1880, vol. i, pp. 27, 323, 356, 381. 



§0n the drift in Brazil. L. Agassiz. Amer. Jour. Sei., 2d ser., vol. xl, 1865, p. 389. A Journey- 

 in Brazil. Professor and Mrs. L. Agassiz. Boston, 18G8, pp. 86-89, 400, 401. Atlantic Monthly^ 

 vol. xviii, July, 1866, p. 50. 



