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L, Lesquereux on the Coal Formations of North America, 35 
when the water was most loaded with sediment, the heaviest 
particles of muddy matter were deposited all along on both sides 
of the river, just where the current began to lose its force; and 
that by this process, continued for a long period of years, a natu- 
ral dam being built along some rivers, the marshes on both sides 
of it, and formerly inundated, were eventually put out of reach 
of the inundations. I have myself ascertained that the thin 
particles of sediment which were at first deposited upon the 
marshes, formed an essential preparation for the growth of the 
t, viz. an impermeable basin, and that it was only when this 
in was entirely isolated and protected against inundation that 
the plants of the peat bogs began to appear and the peat to grow. 
__ This process explains the formation of the fire-clay which under- 
lies every bed of coal. 
The true peat bogs of the Mississippi delta are mostly located 
on or near the old shores of some crooked bayou and surrounded 
on all sides by a kind of embankment. Thus they are free from 
the influence of river water which, though clear, would stop the 
growth of the peat, by destroying the peculiar vegetation of the 
‘he action of the water in building its own banks along the 
principal bed of a river is beautifully exemplified in the United 
States, especially along the Mississippi and some of its tributaries. 
Both sides of the Minnesota river are thus bordered by exten- 
sive marshes which cover the bottom land to the base of the 
ridge of the prairies. In spring they are filled with water, while 
the banks of both sides of the narrow channel are mostly dry 
still high above water. It is then very difficult to cross those 
