DEPOSITION ON CONTINENTAL SHELF AND SLOPE 139 
tinent appear above the ocean... . . A continuous shelf almost 
universally borders the continents..... The continent was 
recently so deformed that these shelves were out of water,” and, 
on a later page: “‘The continental shelf . . . . may be supposed to 
have been built out upon the border of the sea-basin by progressive 
sedimentation.’ 
The meaning of all these writers is clearly that a continental 
shelf—a constructional feature, built of marine sediments—has been 
uplifted and dissected and then resubmerged. 
In textbooks the statement is frequently made that the true 
boundary of a continent is situated at the outer edge of the conti- 
nental shelf—the “continental edge’ of Murray—rather than at the 
shore line.? At first sight this reads like a negation of the view that 
the shelf is mainly a built feature of recent growth. Such state- 
ments seem, however, generally intended merely to convey the 
information that, when mean slopes of the earth’s surface are 
plotted as a hypsographic curve, there is a sharp change of average 
slope at the depth of the edge of the continental shelf but none at 
sea-level. 
THE PRESENT-DAY SHELF LARGELY CONSTRUCTIONAL 
Since ‘‘rival”’ hypotheses of erosion, marine and subaérial (fol- 
lowed by subsidence), and deposition have been put forward by 
Lake, Gardiner, and others, it is necessary to examine critically the 
consequences of each. 
The hypothesis that marine erosion is alone responsible for the 
formation of the continental shelf —Undoubtedly wave action is 
capable of eroding, not only at the shore line, but at considerable 
depths, wherever the ratio of the waste supply to the transporting 
power of the sea is sufficiently low to permit parts of the rock 
bottom to be swept clean, and where there is at the same time 
sufficient movement of the water to move particles of appreciable 
t Chamberlin and Salisbury, Geology, III (New York, 1906), 521, 526. 
2 The shelf is described, for example, as ‘“‘submerged parts of the continental pro- 
tuberance”’ (R. D. Salisbury, Physiography, p. 22), and it is stated that “‘the area 
between the actual shore and the 1oo-fathom is regarded as belonging to the con- 
tinents, though at present overflowed by the sea’’ (Hatch and Rastall, The Petrology 
of the Sedimentary Rocks [London, 1913], p. 11). 
