438 A JOURNEY IN BRAZIL. 
seeking their natural level, gradually widening and deep- 
ening the channels in which they flowed, as they worked 
their way down to the sea. When they reached the shore, 
there followed that antagonism between the rush of the 
rivers and the action of the tides, between continental 
outflows and oceanic encroachments, which still goes on, 
and has led to the formation of our Eastern rivers, with 
their wide, open estuaries, such as the James, the Potomac, 
and the Delaware. All these estuaries are embanked by 
drift, as are also, in their lower course, the rivers con- 
nected with them. Where the country was low and flat, 
and the drift extended far into the ocean, the encroach- 
ment of the sea gave rise, not only to our large estuaries, 
but also to the sounds and deep bays forming the most 
prominent indentations of the continental coast, such as 
the Bay of Fundy, Massachusetts Bay, Long Island Sound, 
and others. The unmistakable traces of glacial action upon 
all the islands along the coast of New England, sometimes 
lying at a very considerable distance from the main-land, 
give an approximate, though a minimum, measure of the 
former extent of the glacial drift seaward, and the sub- 
sequent advance of the ocean upon the land. Like those 
of the harbor of Para, all these islands have the same 
geological structure as the continent, and were evidently 
continuous with it at some former period. All the rocky isl- 
ands along the coast of Maine and Massachusetts exhibit 
the glacial traces wherever their surfaces are exposed by the 
washing away of the drift ; and where the drift remains, 
its character shows that it was once continuous from one 
island to another, and from all the islands to the main-land. 
It is difficult to determine with precision the ancient 
limit of the glacial drift, but I think it can be shown 
