174 
PHYSICAL HISTORY OF THE 
Yalley during the periods immediately follow¬ 
ing the cretaceous, we know little or nothing. 
Whether the tertiary deposits are hidden un¬ 
der the more modern ones, or whether they 
are wholly wanting, the basin having, perhaps, 
been raised above the sea-level before that 
time, or whether they have been swept away 
by the tremendous inundations in the valley, 
which have certainly destroyed a great part of 
the cretaceous deposit, they have never been 
observed in any part of the Amazonian basin. 
Whatever tertiary deposits are 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 Yal¬ 
ley of the Amazons is by no means an eas} r 
task, and its difficulty is greatly increased by 
the fact that the lower formations are only 
accessible on the river margins during the 
vasante , as it is called, 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, 
