VALLEY OF THE AMAZONS. 
were those when the waters are lowest,— 
reaching their minimum in September and 
October, and beginning to rise again in No¬ 
vember,— so that I had an excellent oppor¬ 
tunity in ascending the river to observe its 
geological structure. Throughout its whole 
length, three distinct geological formations 
may be traced, the two lower of which have 
followed in immediate succession, and are con¬ 
formable with one another, while the third 
rests unconformably upon them, following all 
the inequalities of the greatly denudated sur¬ 
face presented by the second formation. Not¬ 
withstanding this seeming interruption in the 
sequence of these deposits, the third, as we 
shall presently see, belongs to the same series, 
and was accumulated in the same basin. The 
lowest set of beds of the whole series is rarely 
visible, but it seems everywhere to consist of 
sandstone, or even of loose sands well strati¬ 
fied, the coarser materials lying invariably 
below, and the finer above. Upon this lower 
set of beds rests everywhere an extensive de¬ 
posit of fine laminated clays, varying in thick¬ 
ness, but frequently dividing into layers as 
thin as a sheet of paper. In some localities 
