138 nea) C. A. COTTON 
The waves move material along the bottom and prevent the settling of the 
finest silt until the limit of wave action is reached. Beyond that limit the 
bulk of the material is rapidly deposited from suspension. In protected 
situations this depth becomes less and in many places is not over fifty fathoms. 
There is thus built outwards around the continents a subaqueous terrace, its 
top gently sloping to a depth of too fathoms or less, its front much steeper in 
comparison, and giving sharpness to the continental margin." 
Unqualified support of any hypothesis which assigns the for- 
mation of the shelf entirely to erosion is rarely met with. In an 
explanation of the shelf given by Mill,? however, marine erosion 
with stationary sea-level is given first place, and even in an Ameri- 
can textbook submergence of a plain “‘worn to low relief” is the i 
only explicit explanation given. That this statement does not 
really express this author’s view is shown, however, by the following 
passage, which appears on another page in an explanation of coastal 
plains (p. 508): ‘‘Off the eastern coast of United States there is 
a level sea-bottom plain, known as the continental shelf. ... . it 
there should be an uplift of 600 feet, this very level plain would 
be added to the continent. .... It would be underlain by un- 
consolidated sediments.” 
The following clear statement on the subject by Gilbert and 
Brigham is also worthy of attention as an emphatic refusal of sup- 
port to the hypothesis of submergence of a plain of erosion: 
It will be remembered that the resemblance of Chesapeake Bay to a 
branching river was explained by saying that the Coastal Plain had sunk down 
so as to let the sea flow into the Susquehanna Valley. Because we now point 
out that the plain is an old sea-bed which has risen, it must not be thought 
that one fact contradicts the other. Both changes have taken place, but at 
different times. After the plain had been formed under the sea it was lifted 
so high that rivers dug deep valleys across it; then it was lowered part way, 
to the present height.4 
A somewhat similar statement from Chamberlin and Salisbury 
may be quoted: ‘‘Almost nowhere does the real edge of the con- 
«J. Barrell, “The Upper Devonian Delta of the Appalachian Geosyncline,” Am. 
Jour. Sci., XXXVI-XXXVII (1913-14), 249. 
2H. R. Mill, The Realm of Nature (2d ed.; London, 1913), p. 219. 
3R. S. Tarr, College Physiography (New York, 1914), p. 641. 
4 Gilbert and Brigham, An Introduction to Physical Geography (New York, 1902), 
p. 153. 
