262 J. C. BRANXER — DECOMPOSITION OF ROCKS IN BRAZIL. 



Some of the most interesting and most impressive cases of rock decay 

 that have fallen under my observation or that I have been able to learn 

 about are to be found in the gold mines of Minas Geraes. 



The old rock mines of the Portuguese and Brazilian miners were almost 

 without exception in decomposed rocks. These early miners, however, 

 probably never worked mines deeper than about 100 feet. 



At Sao Joao da Chapada, about 16 miles west of Diamantina, Minas 

 Geraes, the diamond mines are in beds of schists * so decomposed that 

 they stand only at the angle of repose for claj^s or other soft materials. 

 This material has been penetrated to a depth of from 65 to 90 feet with- 

 out hard rock being reached.f 



In sinking a new shaft at the Morro Velho mine, in Minas, in 1868-'69, 

 " the ground 23roved jointy and unfavorable for sinking for the first 12 

 or 13 fathoms, after which it became harder and more compact," though 

 not yet firm. J The first four montlis' work was in unfavorable ground 

 (that is, decomposed rock), and as the average given was 20 feet a month, 

 at 80 feet the rock was not yet hard. The ])U\n and sections of the mines 

 exhibiting the workings for January 31, 1876, show that one of the shafts 

 was tim])ered to a depth of 126 feet, which is the depth of decayed rock 

 at this point. 



The present superintendent of the Morro Velho mine, Mr George Chal- 

 mers, has kindly written me as follows upon this subject: " In sinking the 

 shafts we found it (the clay-slate) quite soft to a depth of 25 fixthoms 5 

 feet (155 feet). It then turned to blasting rock." He thinks the decom- 

 position does not exceed 30 fathoms on the Morro Velho propert\^ The 

 same gentleman writes me that at the Raposas mine the ore body and 

 some of the country rock are decomposed in certain phices to a depth of 

 200 feet.g 



At the Faria mine, near Congonhas de Sahara, a depth of 164 feet has 

 been penetrated. The rocks are soft schists, " sometimes running into 

 genuine clays." || 



Prior to 1825 the old Catta Preta mines, near Inficionado and about 



*Sur les gisements diamantif6res de Minas Geraes (Br6sil). (Jorceix. Comptes Rendus, 

 xeiii, 1881, p. 982. Observafoes sobre algumas rochas diamantiferas de Minas Geraes. Pelo Dr 

 O. A. Derby. Arehivos do Mus. Nat., iv, 1879. Rio de Janeiro, 1881, p. 127. Explorations of tlie 

 Highlands of Brazil. R. F. Burton. London, 1869, vol. ii, pp. 129-1.32. 



t In his monograph on the diamond M. E. Boiit<\n (p. 134) says that this mine is 40 meters deep- 

 He gives a plate (ii), however, made from a photograph which shows that this is a mistake— that 

 the depth is as stated by Derby. 



I Thirty-ninth An. Rep. Saint John del Rey Mining Company, 1869, pp. 5, 6; Fortieth Report, pp. 

 5-7. 



§ Private letter, dated Morro Velho, August 3, 1895. 



Ii L'or a Minas Geraes. M. Paul Ferrand. Ouro Preto, 1804, i, p. 150. Notice sur la Mine d'or 

 de Faria. Situation au l^r Jan., 1894. 



