18 THE OCEAN. 
creased, as is particularly the case at the mouths of 
the Ganges and Mississippi; while from the same 
cause the bottoms of inland seas being gradually 
raised, the water rises in the same proportion and 
encroaches on the land. The port of Ravenna, once 
a rendezvous for the Roman fleets, has been filled 
up by the deposition of the Montone, a small river, 
so that now it is four miles from the sea.: On the 
other hand the palace of the Emperor Tiberius at 
Capreea, on the opposite shore of Italy, is now wholly 
covered by the water: nor are our own coasts, and 
especially those of Holland, deficient in examples of 
once fertile fields, which are now rolled over by the 
tide. 
Much ignorance prevails respecting the depth of 
the Ocean: in many places no length of sounding 
line has yet been able to reach the bottom, and, 
therefore, our conclusions must be formed from in- 
ference or indirect evidence. Generally, where a 
coast is flat and low, the water is shallow for a con- 
siderable distance, slowly deepening; on the other 
hand, s high and mountainous coast usually is 
washed by deep water, anda ship may lie almost 
close to the rocks. From these circumstances, as 
well as from the various depths actually observed by 
sounding, it is probable that the average depth of 
the sea is not greater than the height of the land, 
in proportion to its extent. If we were to place 
a thick coating of wax over the bottom of a dish, 
taking care to make a very irregular surface, with 
cavities and prominences of all forms and sizes, we 
should probably have a fair idea of the solid surface 
