n
the river can build it up. The great island of Marajo was originally a
continuation of the Valley of the Amazons, and is identical with it in
every detail of its geological structure. My investigation of the island
itself, in connection with the coast and the river, leads me to suppose
that, having been at one time an integral part of the deposits described
above, at a later period it became an island in the bed of the Amazons,
which, dividing in two arms, encircled it completely, and then, joining
again to form a single stream, flowed onward to the sea-shore, which in
those days lay much farther to the eastward than it now does. I suppose
the position of the island of Marajo at that time to have corresponded
very nearly to the present position of the island of Tupinambaranas,
just at the junction of the Madeira with the Amazons. It is a question
among geographers whether the Tocantins is a branch of the Amazons, or
should be considered as forming an independent river system. It will be
seen that, if my view is correct, it must formerly have borne the same
relation to the Amazons that the Madeira River now does, joining it just
where Marajo divided the main stream, as the Madeira now joins it at the
head of the island of Tupinambaranas. If in countless centuries to come
the ocean should continue to eat its way into the Valley of the Amazons,
once more transforming the lower part of the basin into a gulf, as it
was during the cretaceous period, the time might arrive when
geographers, finding the Madeira emptying almost immediately into the
sea, would ask themselves whether it had ever been indeed a branch of
the Amazons, just as they now question whether the Tocantins is a
tributary of the main stream or an independent river. But to return to
Marajo, and to the facts actually in our possession.

The island is intersected, in its south-eastern end, by a considerable
river called the Igarape Grande. The cut made through the land by this
stream seems intended to serve as a geological section, so perfectly
does it display the three characteristic Amazonian formations above
described. At its mouth, near the town of Soure, and at Salvaterra, on
the opposite bank, may be seen, lowest, the well-stratified sandstone,
with the finely laminated clays resting upon it, overtopped by a crust;
then the cross-stratified, highly ferruginous sandstone, with quartz
pebbles here and there; and, above all, the well-known ochraceous,
unstra