1879.] | 279 [Shaler. 
cation of the existence of a precisely similar shelf along the shores of 
the Mediterranean Sea, or the great inland lakes of this continent. 
Along ail tideless shores there is generally a fringe of detrital matter 
extending out a certain distance from the coast. This shelf in tide- 
less seas differs from that we find along tide-swept shores in certain 
important ways. It is much less wide, and has its outer edge in very 
- much shallower water. 
Tf I am right in my observations this difference between the detri- 
tal shelf in tideless and tidal seas, is due to the action of the tidal 
currents -which are active along the shores of the ocean. It is a 
well known fact that the vigor of the movement of the tides dimin- 
ishes with the increase in the depth of the water, and at one hundred 
fathoms the power of the tidal currents must be very small. There is 
but one force working along the shores of the tideless seas, compe- 
tent to convey detritus seaward, the force of the under-tow arising 
from the current driven against the shore by the winds, while along the 
shores of the open seas this current, in itself stronger by virtue of 
the heavier waves, is aided by the tidal action which is probably 
much more effective than the wave-current in transporting substances 
at considerable depths. While it is to the waves that we must look 
for the effective work of breaking down the shores of the sea, and 
grinding up the detritus, it is to the tides that we mostly owe the 
transporting and sorting of this sediment over the detrital plane of 
the coast-shelf. 
__ The action of tidal and undertow currents in removing sedimentary 
matter seaward is doubtless materially aided by the motion of the 
waves; even at a depth of one hundred fathoms the vertical oscilla- 
tion of the waves would be sufficient to disturb fine silt and put it in 
the control of the currents produced by the tides and the undertow. 
When the sea is gaining on the land, this shelf is generally less 
distinctly determined than when the land is rising; the process of ele- 
vation is very favorable to the rapid building of a well-marked detri- 
tal shelf, for, while it is going on, the sea obtains control of much 
more detrital matters than when the shore is stationary or subsiding. 
Ido not care at present, to discuss the history of this interesting 
accumulation of ditritus; that cannot be effectively done without the 
use of more data than can be furnished in this paper; it will be 
worth while, however, to consider some of the geological consequences 
arising from the rapid movements of these great masses of detrital 
