216 
PHYSICAL HISTORY OF THE 
pared with the Amazons, — the Mississippi, 
the Nile, the Ganges, and the Danube, — de¬ 
posit extensive deltas ; and the smaller rivers 
also, with few exceptions, are constantly build¬ 
ing up the land at their mouths by the mate- 
rials they bring along with them. Even the 
little river Kander, emptying into the Lake of 
Thun, is not without its delta. Since my re¬ 
turn from the Upper Amazons to Para I have 
made an examination of some of the harbor 
islands, and also of parts of the coast, and have 
satisfied myself that, with the exception of a 
few small, low islands, never rising above the 
sea-level, and composed of alluvial deposit, they 
are portions of the mainland detached from 
it, partly by the action of the river itself, and 
partly by the encroachment of the ocean. In 
fact, the sea is eating away the land much 
faster than the river can build it up. The 
great island of Marajo was originally a contin¬ 
uation of the Valley of the Amazons, and is 
identical with it in every detail of its geologi¬ 
cal 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 deposiis 
