16 A JOURNEY IN BRAZIL. 



materials to powder ? We know that such an agency has 

 been at work on the northern half of this hemisphere. We 

 have now to look for its traces on the southern half, where 

 no such investigations have ever been made within its warm 

 latitudes ; though to Darwin science is already indebted for 

 much valuable information concerning the glacial phenomena 

 of the temperate and colder portions of the South American 

 continent. We should examine the loose materials in every 

 river we ascend, and see what relation they bear to the dry 

 land above. The color of the water in connection with the 

 nature of the banks will tell us something. The waters of 

 the Rio Branco, for instance, are said to be milky white ; 

 those of the Rio Negro, black. In the latter case the color 

 is probably owing to the decomposition of vegetation. I 

 would advise each one of our parties to pass a large amount 

 of water from any river or stream along which they travel 

 through a filter, and to examine the deposit microscopically. 

 They will thus ascertain the character of the detritus, 

 whether from sand, or lime, or granite, or mere river mud 

 formed by the decomposition of organic matter. Even the 

 smaller streams and rivulets will have their peculiar char- 

 acter. The Brazilian table-land rises to a broad ridge 

 running from west to east, and determining the direction 

 of the rivers. It is usually represented as a mountain 

 range, but is in fact nothing but a high flat ridge serving as 

 a water-shed, and cut transversely by deep fissures in which 

 the rivers flow. These fissures are broad in their lower 

 parts, but little is known of their upper range ; and whoever 

 will examine their banks carefully will do an important 

 work for science. Indeed, very little is known accurately 

 of the geology of Brazil. On the geological maps almost 

 the whole country is represented as consisting of granite. 



