56 PARA. Cuap. III. 
existence? It is difficult to decide the question; but the existing 
evidence goes far to show that it has not. If the mouth of the great 
river, which, for a long distance, is 170 miles broad, had been originally 
a wide gulf, and had become gradually filled up by islands formed of 
sediment brought down by the stream, we should have to decide that 
an effectual barrier had indeed existed. But the delta of the Amazons 
is not an alluvial formation, like those of the Mississippi and the Nile. 
The islands in its midst and the margins of both shores have a founda- 
tion of rocks, which lie either bare or very near the surface of the soil. 
This is especially the case towards the sea-coast. In ascending the 
river southward and south-westward, a great extent of country is 
traversed which seems to have been made up wholly of river deposit, 
and here the land lies somewhat lower than it does on the sea-coast. 
The rocky and sandy country of Marajo and other islands of the delta 
towards the sea, is so similar in its physical configuration to the 
opposite mainland of Guiana that Von Martius concluded the whole 
might have been formerly connected, and that the Amazons had forced 
a way to the Atlantic through what was, perhaps, a close series of 
islands, or a continuous line of low country. 
