536 ORVILLE A. DERBY 



region. In part the predominant rocks are of a totally differ- 

 ent character representing in general terms the so-called crystal- 

 line schists with the exclusion of the true gneisses, and taken as 

 a whole, it may perhaps be assumed that they are more suscep- 

 tible to decay than the rocks of the coastal region in which 

 gneiss and granite predominate. The comparison to be a fair 

 one should be made between campo and forest region consti- 

 tuted by rocks of, as nearly as may be, the same character. 

 The only elements for such a comparison are the superficial 

 observations from the car window in traveling over the Central 

 Railroad. In the long stretch of line in the forested region from 

 Rio de Janeiro over the Serra do Mar and along the Parahyba 

 and Parahybuna valleys to the crest of the Mantiqueira range 

 one seldom fails to see sound rock, gneiss or granite or both, in 

 the bottom (and often in the entire side) of cuttings that exceed 

 a moderate depth. After passing the crest of the Mantiqueira 

 and entering the campo region of Barbarcena sound rock is the 

 exception rather than the rule in the cuttings though these con- 

 tinue as numerous and deep as in the other section and are also, 

 for the most part, in gneiss and granite. The appearance, pos- 

 sibly deceptive, is that decomposition is even more profound in 

 the campo than in the forest region. One plausible explanation 

 for this difference is topographical, since in the forest region the 

 road mainly follows the valleys and is consequently near drain- 

 age level whereas in the campo region it is more independent of 

 the streams and is in general considerably above that level 

 The rocks also, although of the gneiss and granite type, are 

 apparently more acid in composition with a greater abundance 

 of free quartz and of potash mica while magnesia, mica and 

 amphibole are the predominant bisilicates in the rocks of the 

 forest region. There are many indications that this difference 

 in composition is accompanied by a greater susceptibility to 

 decay although exact observations on this point have not been 

 recorded. The differences noted in topographical structure and in 

 rock composition may account for the apparently greater decay 

 in the campo region and in that case the decay in the two regions 



