410 
PHYSICAL GEOGRAPHY OF 
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 dis- 
tant 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 1320 feet above the sea: this is as near 
as possible 2000 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 Martins is 670 
feet, we shall find, the distance being about 1400 miles, 
that the rise is only five and a half inches per mile. If 
we had the height of Barra do Bio Negro accm*ately, 
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 seven hundred miles, and we may 
therefore probably estimate the height at less than 200, 
and perhaps not more than 150 feet. 
