ers of
Patagonia and the southern parts of the Argentine Republic. Not only is
there this general correspondence between the mountain elevations and
the river systems, but as the larger river basins of North
America--those of the St. Lawrence, the Mississippi, and the
Mackenzie--meet in the low tracts extending along the foot of the Rocky
Mountains, so do the basins of the Amazons, the Rio de la Plata, and the
Orinoco join each other along the eastern slope of the Andes.

But while in geographical homology the Amazons compare with the St.
Lawrence, and the Mississippi with the Rio de la Plata, the Mississippi
and the Amazons, as has been said, resemble each other in their local
geological character. They have both received a substratum of cretaceous
beds, above which are accumulated their more recent deposits, so that,
in their most prominent geological features, both may be considered as
cretaceous basins, containing extensive deposits of a very recent age.
Of the history of the Amazonian Valley during the periods immediately
following the Cretaceous, we know little or nothing. Whether the
Tertiary deposits are hidden under 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 Valley of the Amazons 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_, 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, 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 to observe its geological structure. Throughout its
whole length, three distinct geological formations may be traced, the
tw