AEEAS STUDIED AND THEIR CHARACTERISTICS. 261 



I have seen such gullies in Minas as much as 75 feet deep and possi- 

 bly more than that in some instances. The banks show that the rocks 

 have simply decomposed in place, and the decomposition is often so 

 complete that the entire surface exposed is practically one mass of parti- 

 colored clays. 



Not the least striking thing about many of these gullies is their bright 

 colors and the fantastic forms produced by rapid erosion.* The rocks 

 seem to have been schists for the most part, some of them micaceous and 

 others talcose. Quartz veins are common in them, but the quartz has 

 broken into small angular fragments. But while gullies are washed out 

 with amazing rapidity, the deepest ones I have seen are not on the trails 

 first made by the early travelers more than 100 years ago. The oldest 

 gullies seem to have reached a depth beyond which excavation has been 

 retarded for some reason, and thereafter they widened at the top until 

 the uppermost part of the decayed rock had been removed over an area 

 four or five times the width of the original gully when its principal depth 

 was reached. Some of the most remarkable of these gullies are in the 

 region between the Serra de Mantiqueira and Ouro Branco. 



Natural washouts on the sides of the hills in the campo region, between 

 Sitio and the Ouro Preto mountains, are common in other places than the 

 trails, however. Dent mentions these barrancas^ as they are called, " often 

 100 to 200 feet deep," f between Brumado and Suassuhy. 



Mr Wells thinks these barrancas are land-slides. % Some of them 

 doubtless are, but certainly not all of them. 



Liais states that in about 40 years between 300,000 and 400,000 cubic 

 meters of earth were removed from one of these barrancas. Some of the 

 barrancas he thinks are land-slides. § 



Castelnau thinks the land-slides may have been caused partly by earth- 

 quakes. II 



* Burton's Highlands of Brazil, i, p. 74. Sud-Amerique. Charles d'Ursel. Paris, 1879, p. 55. 



t A year in Brazil. H.C.Dent. London, 1886, p, 37. 



X Three Thousand Miles through Brazil. J. W. Wells. London, 1886, vol. ii, p. 372. 



§ Climats, etcetera, p. 4. 



II Expedition dans I'Amerique du Sud., i, p. 202. The barrancas about Barbacena have attracted 

 more attention than those of any other part of the country, because Barbacena is on the highway 

 leading from Rio de Janeiro to the gold and diamond region of Minas. 



For other cases of deep decomposition see : 



Penedos de Dioritos do Valle do Parahyba do Sul. Comte de la Hure. Revista do Instituto 

 Historico do Brazil, xxix, 1866, pp. 422-429. 



Hartt's Geology and Physical Geography of Brazil, pp. 145, 159. 



Eeise in Brasilien, Spix u. Martins. Munschen, 1823, i. p. 302. 



Travels in South America. Alexander Caldcleugh. London, 1825, vol. ii, pp. 192, 210, 213, 215, 227, 

 229, 230, 258, 260, 282. 



Beitrage zur Gebirgskunde Brasiliens. Joh. Em. Pohl. Wien, 1832, pp. 26-28. American Natural- 

 ist, Sept., 1884, vol. xviii, p. 927. 



Ueber das Geognostisehe Verkommen der Diamanten. V. von Helmreichen. Wien, 1846, pp. 

 5,7, 12,15. 



