PROFILE OF THE SUBAQUEOUS SHORE TERRACE 23 
pairs of agencies are in conflict as to the direction in which 
bottom materials are to be moved. If all the water which 
moves shoreward must return over the same area and as a bot- 
tom current, this current would seem to have greater efficiency 
than the one above, moving in the opposite direction. This is 
certainly the case where translatory waves are not favored, as 
where the off-shore slope is steep. Where slope is gentle and 
translatory waves are well developed, they have one decided 
advantage. They are short as compared with the distance from 
wave to wave, hence all the shoreward movement of the water 
is concentrated into a small portion of the entire time. Divers 
are said to feel the passing of one of these waves as a sudden 
jerk between intervals of quiet. The undertow, on the other 
hand, has a steady flow except as interrupted by these sudden 
reverses.’ The laws of energy give to these concentrated move- 
ments a much greater efficiency than to the same amount of 
motion more evenly distributed in time. On many shores of 
gentle slope, sand is worked landward, and in this process the 
agency just mentioned is doubtless important. The effect here 
referred to is that of waves of translation and is therefore inside 
the breaker line. It might accumulate sand on-shore but not 
in off-shore barriers. The dominance of shoreward action is 
essentially temporary (omitting currents alongshore from con- 
sideration). Its effect is to steepen by narrowing the slope. 
This steepening, in turn, is adverse to waves of translation. 
Laws of equilibrium, eroding currents —Ignoring the presence 
of a bank and the load derived from it, a current of uniform 
power tends to reduce the bottom to a level surface, that is, to 
require equal depth throughout. Equilibrium cannot exist on a 
level bottom where the power of the current is unequal at dif- 
ferent places. In such cases, the depth must suffer a corre- 
sponding change until the power of water on the bottom is 
HENRY MITCHELL, “On the Reclamation of Tide-Lands and its Relation to 
Navigation,” Report of the U.S. Coast and Geodetic Survey, 1869, Appendix 5, p.85. 
In this paper Mr. Mitchell takes the extreme view that the sea restores to the conti- 
nent “all the material washed from its bluffs and headlands.”’ Certain exceptions are 
made for islands. 
