252 MR. O. A. DKRBY ON NEPnELINE-ROCKS IN BRAZIL. 



placed on the main ridge of the range. At its back two small, but 

 deep, longitudinal valleys — the Sao Pedro and Santa Anna — split the 

 range up into three parallel ridges, of which the third is the main 

 watershed between the river Parahyba nnd the sea, while the other 

 two break down on the seaward side in the immediate vicinity of the 

 peak. All of these are normal gneiss ridges. The outer two are 

 narrow, vt^ith subparallel margins and a sharp angular crest, with a 

 very uniform elevation of 800-900 metres. Near the extremity of 

 the outer or Sao Pedro ridge, the uniformity of the crest is broken 

 by the abrupt elevation of a saddle-like peak, which breaks down 

 with the same abruptness to the normal elevation of the gneiss ridge 

 on the other side. The eruptive mass, whose limits are approxi- 

 mately shown in the sketch-map (fig. 1), extends but slightly to the 

 northward of the crest, and produces no prominent deflection of 

 the inner margin of the gneiss ridge, or of the course of the river 

 Sao Pedro that marks that margin. On the front side, however, it 

 presents an enormous protuberance, which gives the peak, when 

 seen from a distance, the appearance of an independent mass 

 standing in front of the gneiss ridge, instead of resting upon and 

 rising above it. 



Seen from the Sao Pedro side, the peak presents a steep, straight, 

 bastion-like face in the same plane as that of the gneiss ridge, with 

 two prominent conical points — the peak proper and the lower Santo 

 Antonio peak, united by a sort of curtain. On the opposite side, 

 each of these points is seen to be at the junction of secondary trans- 

 verse crests with the main crest. Of these, the one extending from 

 the main peak is the most important, and is crowned by a number 

 of minor peaks, one of which projects towards a lateral ridge of the 

 Santo Antonio peak in such a manner as to nearly enclose a deep 

 amphitheatre-like central depression — the upper valley of the E,io 

 do Ouro. Erom certain points of view, the aspect of this valley is 

 exceedingly suggestive of a crater, which, taken in connexion with 

 the general conical form of the mass and the character of its erup- 

 tive rocks, is probably not without significance. 



The river Sao Pedro passes the back of the peak at an elevation 

 of 600-700 metres, but descends rapidly, and reaches a low marshy 

 plain but slightly elevated above sea-level before escaping past the 

 end of the outer gneiss ridge. This plain, of very recent formation, 

 stretches along the front of the Serra do Mar, from the Bay of Rio 

 de Janeiro on one side to the sea on the other. It is drained 

 by the rivers Iguassii, flowing to the Bay of Eio, and Guandu, 

 flowing to the sea. The former — a sluggish lowland river — receives, 

 through several small streams, the drainage of the broad south- 

 eastern flank of the Tingua peak. The latter issues from a broad 

 gap near the centre of the outer rim of a considerable canoe-shaped 

 longitudinal valley between parallel ridges of the Serra do Mar, of 

 which the river Santa Anna occupies the eastern half. It receives 

 the Sao Pedro from the back of the peak, and the Rio do Ouro and the 

 Santo Antonio from the front. The most characteristic stream of 

 the peak proper is the Rio do Ouro, coming from the central crater- 



