AGENTS AT WORK IN THE OCEAN 231 
driven in as far as they can be moved; but since the 
force of the waves decreases in these narrow, protected 
re-entrants, a place is soon reached where all but the 
finer particles must be deposited. So a bar is com- 
menced, and this may later grow into a beach, which 
may completely extend across the mouth of the bay 
(Fig. 151). This beach 
then becomes the mill 
on which the waves, in 
more leisurely fashion, 
grind up the frag- 
ments that were wres- 
ted from the cliffs. 
Action of the Tides. 
— Twice each day, by 
the combined effect of 
the attraction of sun 
and moon, the surface Fra. 130. 
of the ocean rises and Indentation in coast of Cape Ann, cut by 
waves which have removed a dike rock from 
falls (Fig. 139). In the more durable granite walls. 
some places the range 
between high and low tides is only a foot or two; in 
others it is ten or twelve feet ; and in one or two places 
it reaches a height as great as forty or fifty feet. 
Even in neighboring bays, the tidal range may differ 
by several inches or a foot; and if these bays are con- 
nected by a strait, the difference in the height of the 
