66 CAUSES OF GEOLOGICAL CHANGES. 
which lies along the bay of Biscay, and at a few points on the 
Atlantic coast of the United States. 
2. Rivers; which act upon their own beds, and also carry 
down to the sea the finer particles of earth thai are -washed 
into them from the hills amongst which they flow. 
The rocks and stones in the beds of all streams that flow with 
any considerable degree of rapidity, are tumbled by the current 
upon each other, and gradually diminished in size and worn into 
a round form by the attrition they undergo. All streams are 
rendered turbid by earthy matter, brought in by water that has 
fallen in rain upon the tract drained by them, at some season of 
the year The fine sand and clay they hold suspended are 
deposited at their mouths, forming a Delta or body of alluvial 
ground, which is periodically extended, and with a considerable 
degree of uniformity. The progress of these changes is matter of 
observation and history. " Many cities," says Cuvier, "which 
were flourishing sea-ports in well known periods of history, are 
now several leagues inland, and several have even been ruined by 
this change." We learn from Strabo that Ravenna stood among 
lagunes, in the time of Augustus, as Venice does now; but Ra- 
venna is now at the distance of a league from the sea. Adria, 
which gave name to the Adriatic, was somewhat more than twenty 
centuries ago the chief port of that sea, from which it is now at 
the distance of eighteen miles. The whole of the north-western 
border of the Adriatic, where it receives the streams that flow 
down from the Alps, through an extent of one hundred miles, and 
from two to twenty miles in breadth has been gained from the 
sea, and converted into a low marsh within the last two thousand 
years. The cities, Rosetta and Foah, built originally at the 
points where the lower branches of the Nile join the Mediterra- 
nean, afford the same evidence of the extension of the land, being 
now at some distance from the sea. The Delta of the Ganges and 
Burrampooter, extends two hundred miles along the coast, and 
not less than two hundred and twenty into the interior. That 
of the Mississippi is constantly advancing into the Gulf of Mexico. 
In the formation of a Delta, the sediment is thrown down first 
and most abundantly at the mouth, and on the immediate bank of 
the stream, and a long narrow ridge is created, the middle of which 
is occupied throughout its length by the river channel. The 
declivity of the plane along which the water flows, being dimin- 
ished by an increase in its length, whilst its altitude remains the 
same; the sediment is also accumulated at the bottom, which is 
thus raised, and the stream may be described as running upon a 
ridge, with a tendency becoming constantly greater to pass its 
banks and deluge the country on either hand. The Mississippi 
exhibits these results in the lower part of its course. The ridge 
formed by its alluvion is about three miles in width, including the 
stream, and twenty-four feet high; but as the river is from one 
hundred to one hundred and sixty feet deep, its bottom is far be- 
