DECOMPOSITIOX OF ROCKS IN BRAZIL 55 5 



texture lhroUL!,iiout a considerable thickness and, so far as can be 

 judged from a simple inspection, should be one of the least sus- 

 ceptible to decay of the series of metamorphic schists to which 

 it belongs. The notes given by Branner refer to two sets of 

 shafts which should be carefully discriminated. The first were 

 sunk at the bottom of a gorge, possibly a fault line, and thus 

 were almost wdioUy below drainage level. The term "jointy" 

 applied in the mine report to the first dozen fathoms would 

 hardly be used by miners for a decomposed condition of the 

 rock so that the depth of timbering (126 feet) is probably a 

 measure of the depth to which the rock was shattered, perhaps 

 through faulting, rather than decay. The new shafts, to which 

 the information given by the present superintendent refers, are 

 so situated that the depth at which "blasting rock" was found 

 affords a good measure of the normal decay of this class of 

 rocks above drainage level. They are on the side of a campo- 

 covered ridge wath a slope of 20° to 30° to the nearest stream 

 some 300 feet below the mouth of the shafts. Decomposition 

 here, though profound (155 feet), is still far above drainage 

 level and probably under the same circumstances the more per- 

 meable rocks of the same series (especially those in which rapid 

 alternations of layers of different composition and texture occur) 

 would be found decomposed to a much greater depth. Thus in 

 the adjoining Faria mine the depth cited (164 feet) probably 

 refers to the depth of working and not to that of decay which 

 under the circumstances of the mine and of its rock may be very 

 much greater. On the other hand the soft bed intercalated in 

 hard rock reported at Morro Velho at a depth of 755 feet could 

 hardly have been a case of normal decay. So also the soft 

 (friable) rock reported in the Cocaes (300 feet) and Gongo 

 Socco (375 feet) mines probably do not represent decomposi- 

 tion as it is very doubtful if the rock of these mines (itabirite) 

 was ever in a compact condition. 



In traveling through the mining districts of Minas Geraes, 

 which arc mainly in campo, one gets the impression that rock 

 decomposition is even more profound than in the forested coastal 



