140 Car COMHON 
size across the exposed rock. ‘This is an accepted principle. Wave 
motion extends, it is believed, to a depth of about 100 fathoms in 
the open ocean; it has been shown to be sufficiently strong to move 
gravel at a depth of 36 fathoms.’ There is thus no reason to believe 
that, zf the bottom were kept swept clear of sediment, erosion could 
not take place—though, obviously, the slowness of erosion would 
increase with depth—as far seaward as the edge of the shelf in 
100 fathoms, at which depth there would be an abrupt change of 
slope from that of the shelf to that of the initial sea floor. This is 
the gist of the hypothesis as put forward by Mill and by Lake. 
The postulate of submergence following the cutting of the platform 
introduced by Gardiner is unnecessary. 
The question arises whether clean sweeping of the cut platform 
such as is postulated in the preceding paragraph is possible; and 
it would appear that such a state of affairs must be extremely © 
uncommon if it ever occurs. In the ordinary case deposition of the 
waste produced by the cutting of the platform, together with the 
supply from the neighboring land, must take place seaward of 
the area in which erosion is taking place. It is indeed obvious, as 
has often been pointed out, that the observed continental slope must 
be the surface of the last-formed layer of this sediment and repre- 
sents more or less accurately the slope at which it came to rest. 
It seems highly probable that earlier-formed layers came to rest 
at the same inclination, and that some portion of the shelf is every- 
where composed of such inclined layers. 
The combined processes of erosion and deposition have been 
shown by Davis? and by Fenneman: to produce a “graded profile”’ 
or “profile of equilibrium” from the shore to the edge of the shelf, 
which is a fairly even slope though somewhat concave near shore 
and convex seaward. Fig. 1 illustrates the growth of a shelf by 
this combination of erosion and deposition. It will be seen that, 
1A. R. Hunt, “Formation of Ripple Marks,” Proc. Roy. Soc., XXXIV (1882), 
I-13 (see p. 10). . 
2;W. M. Davis, ‘“‘The Outline of Cape Cod,” Geographical Essays (Boston, 1909), 
Pp. 690-724 (see pp. 700-703). 
3 N. M. Fenneman, ‘“‘The Profile of Equilibrium of the Subaqueous Shore Terrace,” 
Jour. Geol., X (1902), I-32. 
