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 Igarapd Grande. The cut 

 made through the land by this stream seems intended to 



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