204 
ARE SOILS ENRICHED, IMPOVERISHED, [Part III. 
over such vast spaces of space as well as of time , 
they are so liable to disorder from particular ac¬ 
cidents , such as changes of the course of the 
river, extraordinary floods, landslips, earth¬ 
quakes, subsidencies, upheavals, partial destruc¬ 
tion of deltas by the sea, &c. &c., that man, un¬ 
able to see what is or what lias taken place , can 
only speculate on what must be , or what must 
have taken place. 
The sea ends every valley, but never yet began 
one; that is, where there is no delta, when the 
river and the tides have done their utmost, it is 
the sea which prevents the farther deepening of 
the estuary. 
The force of the river, as it dies off in the sea, 
becomes as perfectly horizontal as the force of a 
marine current; and neither of them could form 
an inch of sloping valley. But where there are 
no currents to prevent it, the sea, by stopping 
the longitudinal rush of rivers, allows the de¬ 
posit of deltas, and may thus be said to prolong 
valleys. 
“ Bottle off the sea,” and all estuaries and 
their valleys will be deepened; for the tidal part 
of each river will be a torrent on the brow of a 
mountain. In this case deltas and alluviums 
would disappear ; but they would not u go with 
