MR. 0. A. DERBY ON NEPnELIXE-ROCKS IN BRAZIL. 



253 



like depression. The Santo Antonio drains the small south-western 

 riank, and receives, through the Limeira, the drainage of the short 

 section of the gneiss ridge that projects heyond the peak. Of the 

 streams falling into the 8ao Pedro, the most important is the liarra, 

 whose course marks approximately the western limit of the erup- 

 tive mass on that side. 



For a mountain in the Bra/jlian forest-region, the peak is 

 unusually accessible to geological examination, owing to the deriva- 

 tion of its waters for the supply of the city of Rio de Janeiro. A 

 tram-line, 53 kilometres long, leads from the city to the Uio do 

 Ouro reservoir ; while a branch skirts the south-eastern face to 



Fig. 1. — Sl'etch-map of the Thujua masaif. 



Conceiciio, and another passes around the western end of the gneiss 

 lidge to the river Sao Pedro. Xearly all the streams of the front 

 side, together with the Sao Pedro, have been dammed near the 100- 

 metre level, and connected by pipe-lines, shown by the heavy dark 

 trace on the sketch-map* (fig. 1). The numerous cuttings of the 



* The basis of this sketch is the large map of the streams, roads, and pipe- 

 lines kindly furnished by the waterworks engineers, to whom I am greatly 

 indebted for uuiform courtesy and iiearty co-operation in the course of i.his 

 investigation. To give a better idea of the topographical features, 'JOO-melri? 

 contour-hnes, without pretensions to accuracy, liave been sketched iu from a 



Q. J. G. S. Xo. IfcG. "' s 



