534 ORVILLE A. DERBY 



The Pedregulho hill is an isolated elevation surrounded by 

 tidal swamps and old beach deposits, of the type so well described 

 by the Brazilians as a half orange. It thus presents the most 

 favorable disposition for the preservation of the decomposed 

 material except for the inevitable waste from rain washing and 

 wind action, and (at least since the present topographical condi- 

 tions were established) it has received no material from adjacent 

 higher lands and only its own proper amount of atmospheric 

 waters. The cutting away of the top of the hill for the reservoir 

 foundation exposed the upturned edges of a highly schistose 

 gneiss in which the different layers, rarely more than a few cen- 

 timeters thick, vary greatly in the relative proportions of quartz, 

 feldspar and mica (biotite) and consequently in permeability 

 and susceptibility to decay. On the whole, however, it may be 

 said that the rock represents the most susceptible type of the 

 neighborhood and is in the most favorable conditions for rapid 

 decay. At the depth of sixty-five feet from the original sum- 

 mit of the hill the rock was considered sufificiently firm to build 

 upon though it was nowhere perfectly sound and many of the 

 layers were completely earthy, partial decomposition evidently 

 extended much deeper, and it may even be presumed that in 

 some of the layers it may have extended nearly or quite to 

 drainage level, that is to say to near the base of the hill 225 feet 

 below the original summit. 



In the case of the Morro Velho mine the rock concerned is a 

 hard bluish clay slate, but without well defined slaty cleavage, 

 standing nearly vertically. It is very uniform in character and 



rises to an elevation of 700 to 800 meters above tide. Moucliez's figures are presu-m- 

 ably a sailor's estimate at long range and are not in accord with tw^o aneroid determina- 

 tions which gave 520 and 608 meters. The section and detailed description of the 

 mountain given in the same paper represent, in perfect accord with my own observa- 

 tions in the region, the Cretaceous beds as terminating at some distance from the base 

 of the mountain, at an elevation of about one-ihird of its height and only slightly 

 greater than that of the Tertiary which is stated to be about 200 feet. So far as the 

 present evidence goes, therefore, the original estimate of about 1 00 meters as the 

 maximum elevation of the coastal beds may still stand. As at no great distance the 

 base of a very extensive and apparently very different horizontal series of secondary 

 beds is at an elevation of about 300 meters, faulting is strongly suggested. 



