AREAS STUDIED AND THEIR CHARACTERISTICS. 263 



20 miles north of Ouro Preto, were worked in soft rock to a depth of 

 more than 180 feet.* 



The old English gold mines at Cocaes were at least 300 feet deep, and 

 in the soft, friable, grayish colored micaceous iron-schist.f 



The case of deepest decomposition I have been able to find recorded 

 in Minas Geraes is that shown in the workings of the old Gongo Soco 

 mine. The reports of the company show at a depth of 330 feet that the 

 material was still soft ; J at 372 feet it was remarked § that " very little 

 alteration has takeii place in the level, and we have no alteration to re- 

 mark." 



Dr Gardner, who was at Gongo Soco in 1840, says the greatest depth 

 of the mine at that time was 378 feet, and that the schists were all so soft 

 .as to require strong pillars, || while Castelnau, who visited this mine in 

 1843, remarks that it was worked with the pick. ^ 



The Gongo Soco mine was sunk to a depth of 420 feet in 1844, but I 

 have been unable to find out whether the rocks continued soft at this 

 •depth. ** 



James E. Mills, who studied the geology of Rio Grande do Sul about 

 Lagoa da Maga, writes me that at that place " the hard feldspathic por- 

 phyry is softened ... to a depth of 12 or 15 feet, but the softening 

 is by no means as extensive in this part of Rio Grande do Sul as in 

 Minas." Professor Derby reports borings made in Carboniferous rocks 

 of the basin of Arro3^o dos Ratos, Rio Grande do Sul, which show that 

 the rocks are there decayed to a depth of 318 feet in one place and 393 

 in another.ft This last is the greatest depth of the decomposition of 

 rocks in Brazil actually recorded. 



Hussak says the plateau of schist about Catalao, in the southeast corner 

 of Goyaz, are " for the most part completely decomposed. "JJ 



* First Report of the Imperial Brazilian Mining Company, 182C, pp. 66, 69, 70, 71. 



i" Travels in the Interior of Brazil. George Gardner. P. 489. 



t Twenty-eighth Report of the Directors of the Imperial Brazilian Mining Association. London, 

 1840, Mining Captains' Rep., pp. 41, 49-53. 



§ Thirtieth Report, 1841, pp. 48, 56; Thirty-first Report, 1841, p. 35. The reports of this company 

 also contain many general statements and some measurements of decomposition of the rocks at 

 the Antonio Pereira, Cata Preta and other mines. First Report, 1826, p. 16; Second Report, 1826, 

 pp. 52, 53. 



II Travels in Interior of Brazil. George Gardner. London, 1846, p. 493. 



T[ Expedition dans I'Amerique du Sud. Histoire du Voyage. F. de Castelnau. Paris, 1850, i, p. 

 ^47. 



** L'or a Minas Geraes. M. Paul Ferrand. Ouro Preto, 1894, i, pp. 107, 110. 



It Note on the decay of rocks in Brazil. Amer. Jour. Sci., 3d ser., vol. xxvii, 1884, p. 138. This 

 "boring penetrates the underlying gneiss 59 feet, and it occurs to one that this part of the decay 

 tnay have taken place before the deposition of the overlying sediments. Professor Derby, how- 

 •ever, does not think so. On decay of 300 meters said to have been reported by Pissis see foot-note, 

 p. 260. 



Xt Relatorio parcial da Commissao exploradora do Planalto Central do Brazil. Rio de Janeiro, 

 1893, p. 112. 



t 



