NOTES ON GEOLOGY. 423 



Janeiro in two hours, and caused great erosions at the Mono do 

 Castello and on a multitude of argillaceous hills on the Nichteroy 

 side of the bay. One must have seen the torrents in the mountain 

 regions to understand the powerful part which, in the course of 

 centuries, these phenomena can exercise on the configuration of 

 the soil. I have mentioned how the different divides I worked 

 over (four within about twenty miles) are all about the same height 

 (3200 feet), and how these hills abound in canyons, and are sepa- 

 rated by steep valleys, hundreds of feet deep, and sometimes a 

 mile or more in breadth. Many of the erosions have the testi- 

 mony of ocular demonstration. The Visconde de Prados described 

 to M. Liais how one erosion occurred some forty years ago near 

 Barbecena. This crevasse was about seven acres and a half in 

 area, six hundred yards long, fifty-five yards wide, and over thirty 

 feet deep. Therefore, from four hundred thousand to five hundred 

 ■thousand cubic yards were washed away from this hillside, and 

 the earth excavated entirely disappeared, being carried away by 

 the waters. As these canyons are often from one hundred to three 

 hundred feet deep, they show the power of decomposition by 

 •atmospheric agents on the gneiss, and this is in active operation 

 to-day. 



These phenomena explain the valleys of denudation ; they 

 reveal how watersheds have been changed so that the original 

 tributaries of the Parahyba do Sul oi^ Rio de la Plata may now 

 flow to the Rio Sao Francisco ; and they show how valleys may 

 be formed much more rapidly than might be considered at first 

 sight. It is important to bear in, mind the varying resistance to 

 decomposition of different strata. This is the reason of the 

 irregular and picturesque features so abundant in the mountain- 

 ranges, of which the Organs offer a remarkable example. The 

 name "Organ Mountains'' has been given from the supposed 

 resemblance of its peaks to the pipes of an organ, especially when 

 seen from Rio. In the account of my journey to Petropolis, I 

 have mentioned the vertical walls of rock surrounding the huge, 

 -deep amphitheatre now clothed in virgin forest, which the railway 

 scales triumphantly. The pyramids and masses of rock consist 

 of the harder portions of the original strata, which have ^ been 

 thrust up, like the slates of the Longmynd, in Shropshire, at a 



