412 



A JOURNEY m 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, 



