284 



PHYSICAL GEOGRAPHY OF 



and their whole course in the deep forest ; they flow generally 

 over clean granite rocks and beds of sand, and their streams 

 are gentle, so as not to wear away the soft parts of their 

 banks. 



The Iga, Japura, and Upper Amazon, on the contrary, flow 

 through a long extent of alluvial country, and, having their 

 sources on the slopes of the Andes, are much more liable to 

 sudden floods, and by their greater velocity bring down a 

 quantity of sediment. In fact, it seems clear, that a thorough 

 knowledge of the course of each river would enable us to trace 

 the colour of its waters to the various peculiarities of the 

 country through which it flows. 



With the exception of the streams rising in the Andes, the 

 boundaries of the Amazon basin, or the most distant sources 

 of its tributaries on the north and south, are comparatively 

 little elevated above the level of the sea. The whole basin, 

 with the exception of a very small portion, is one great plain 

 of the most perfect and regular character. 



The true altitude of the source in the Lake Lauricocha has 

 not been ascertained. At Tomependa Humboldt states it to 

 be 1,320 feet above the sea: this is as near as possible 2,000 

 miles in a straight line from the mouth ; so that the average 

 rise is only eight inches in a mile. But if w^e take the height 

 at Tabatinga, on the boundary of Brazil, which, according to 

 Spix and Martius is 670 feet, we shall find, the distance 

 being about 1,400 miles, that the rise is only five and a half 

 inches per mile. If we had the height of Barra do Rio Negro 

 accurately, w^e should no doubt find the rise to that point not 

 more than two or three inches in a mile. The distance is, in 

 a straight line, about 700 miles, and we may therefore probably 

 estimate the height at less than 200, and perhaps not more 

 than 150 feet. 



This height I am inclined to believe quite great enough, from 

 some observations I made with an accurate thermometer, 

 reading to tenths of a degree, on the temperature of boiling 

 water. This instrument I received from England, after leaving 

 Pari. The mean of five observations at Barra, some with river 

 and some with rain-water, gave 212*5° temperature of 



boiling water ; a remarkable result, showing that the barometer 

 must stand there at more than thirty inches, and that unless it 

 is, in the months of May and August, considerably more than 



