PRORLEE: Oe Mie S\OBAOGUE OOS SHORE WTEHRRA CE, 35 
ness is reached. This would mean that currents become unable 
to carry the whole load at fifty or sixty fathoms, and at one 
hundred fathoms or less, become unable to transport anything 
except insuspension. If the factor of transportation in suspen- 
sion did not enter, the front of such a shelf should show the 
subaqueous earth-slope. 
It is commonly assumed as above, that undertow and wave 
agitation lose their efficiency at the same point, the limit of the 
former being determined by that of the latter. Probably this is 
very generally true; moreover, since wave oscillation decreases 
with depth in geometrical ratio at a high rate, and the decrease 
of its agitating power is at a rate measured by the square of this 
same ratio, it may readily be seen that there is a somewhat defi- 
nite horizon below which wave action is ineffective. Such a 
condition is signalized by a somewhat definite limit to the sedi- 
mentary shelf. 
Transportation beyond wave-base.—Vhe undertow may, how- 
ever, be constricted laterally and preserved from dissipation, as 
when the water drifts into a re-entrant curve of the shore; or 
deep currents may result from a system of rebounds. By either 
of these means the power of the lower water may be increased, 
so that at depths greater than that of wave-base sand or even 
gravel may be transported.* | In such cases no break in the pro- 
file may be seen at wave-base. Broad sheets or streaks of sand 
may cover the bottom to depths far beyond this line. Such 
troughs as those of the great lakes, in which all the surface 
water may be drifted simultaneously in one direction, should 
especially favor vertical circulation with vigorous movements 
below. Wave-base of Lake Michigan, where revealed by a 
sharp angle at the edge of a marginal terrace, is sixty or seventy 
feet below the surface; yet around much of its margin, a sand 
covered or gravel covered bottom, concave upward, extends 
outward to several times this depth with little or no evidence of 
change of slope at wave-base.? This is to be expected from the 
tSee H. C. KInAHAN, ‘“‘ The Beaufort’s Dyke off the coast of the Mull of Gal- 
loway,” Proceedings of the Royal [rish Academy, Third Series, Vol. VI, No. 1. 
? Charts of Lake Michigan, War Department. 
