22 
AMERICAN GEOLOGY. 
The line ot soundings and the sand deposits lie in the general 
tide wave which washes the shore of the continent, but which 
acquires a maximum accumulation in the banks which have 
just been spoken of. These great deposits are due mainly to 
conflicting currents, which are here met with, particularly the 
confluent tide wave of the Atlantic with the divergent wave of 
the American coast. 
§ 18 . Local and specific deposits belong to the same agency 
as that producing the great cumulative masses of George’s, 
Newfoundland banks, and Sable island, &c. The tides, as they 
are usually understood, or as they are known to the common 
observer, exert a constructive influence upon the materials con¬ 
veyed to them by river currents. The flood tide, advancing 
into a bay, sound, or estuary, bears onward its burden as has 
been already described. It deposits it along the shore in con¬ 
sequence of its conflict with the bottom and irregular sides of 
the projecting land, forming thereby a sandy, ridgy border of 
greater or less extent, according to the amount of the detrital 
matter it has received. The outward flow, or ebb tide, dis¬ 
tributes the materials in a more central track, but is less effect¬ 
ive than the flood tide: it renders the deeper parts of the bay 
more shoal, as it partially gathers the detritus from the sloping 
shores under the convex crest of the retiring wave in the deeper 
channels. Aided by the river current, detritus and floating 
bodies are moved far out to sea. If the cumulative process were 
confined to the bay or estuary, the bottoms would be raised 
much more rapidly than at present. One of the effects of the 
tide wave is to drive across the mouths of the deep coast indent¬ 
ations and bars of sand. In process of time these bars rise to 
the surface, and inclose a bay, producing thereby a lagoon, 
which becomes first brackish, and afterwards fresh water. This 
fact has' an important bearing in geological reasoning, as it is 
in these reclaimed areas that we find oceanic or pelagic shells, 
estuary, and fresh water, and land remains superimposed upon 
one another. The breaking down of the bar by powerful waves 
may convert an inland lake to a bay of fresh or salt water, 
