412 A JOURNEY IN BRAZIL. 
represented in geological maps of this region are so marked 
in consequence of an incorrect identification of strata 
belonging, in fact, to a much more recent period. 
A minute and extensive survey of the valley of the Ama- 
zons is by no means an easy task, and its difficulty is greatly 
increased by the fact that the lower formations are only 
accessible on the river margins during the vasante, or dry 
season, when the waters shrink in their beds, leaving a great 
part of their banks exposed. It happened that the first three 
or four months of my journey (August, September, October, 
and November) were those when the waters are lowest, 
reaching their minimum in September and October, and 
beginning to rise again in November, so that I had an 
excellent opportunity, in ascending the river, of observing 
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 conformable with one another, while the third rests un- 
conformably upon them, following all the inequalities of the 
greatly denudated surface presented by the second forma- 
tion. Notwithstanding 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 sand- 
stone, or even of loose sands well stratified, the coarser 
materials lying invariably below, and the finer above. Upon 
this lower set of beds rests everywhere an extensive deposit 
of fine laminated clays, varying in thickness, but frequently 
dividing into layers as thin as a sheet of paper. In some 
localities they exhibit, in patches, an extraordinary variety 
of beautiful colors, pink, orange, crimson, yellow, gray, 
