454 HS (Cn (Cla AVE PIS IEIONG 
of Powell, they have not come into that large service as work- 
ing principles of which they are susceptible. 
Every continent which stands in a given position with refer- 
ence to the sea for any prolonged period develops a submarine 
terrace about its borders. This is formed from the débris of the 
land deposited beneath the edge of the sea. In its initial stages 
it is nothing more than the familiar shore terrace; but as it 
develops it becomes a broad submerged platform with a steep 
face dropping away to the abysmal depths of the ocean. The 
submerged platform has its outer limit at the depth at which 
detritus can be effectively moved off shore by the agitation of 
the surface waters. This, though varying with conditions, may 
be roughly taken to be one hundred fathoms. The breadth 
attained by the upper surface of the terrace is conditioned upon 
the length of time the continent remains in a fixed attitude and 
the activity of land wash. Simultaneously the sea cliffs are 
moving inland, and the valleys are developing base plains which 
are the correlatives of the terrace plain which is growing sea- 
ward, as illustrated in Fig. 1. 
Fic. 1.—o-0 Original surface. s/7 Sea level. e Land carried away by erosion. 
@ Detritus built into circumcontinental terrace. ¢ / Terrace plain. a@ ¢ Abysmal 
terrace face. 6 Base-plain developing landward. 
The extreme limit of development is attained when the 
continent has been baseleveled and no farther detritus is fur- 
nished for the extension of the terrace. The baselevel of the 
continent then becomes essentially continuous with the sub- 
merged terrace surface, and the whole constitutes the perfected 
continental platform, as shown in Fig. 2. , 
The development of the circumcontinental terrace and of the 
perfected continental platform is subject to intercurrent disturb- 
