284 rHYSICAL 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 lea, 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 we 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, we 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 Para The mean of five observations at Barra, some with river and some with rain-water, gave 2i2 > 5° as the 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
