PHYSICAL HISTORY OF THE AMAZONS. 433 
logical 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 sup- 
pose 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 con- 
sidered 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 Ma- 
deira 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 southeastern end, by a 
considerable river called the Igarape Grande. The cut 
made through the land by this stream seems intended to 
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