396 
circum-continental shelf is the submarine prolongation of the pene- 
plain above the sea-level '). The seaward growth of the shelf comes 
to an end only as soon as the continent concerned will have been 
eroded to about its base-level, i.e. has become a perfect peneplain. 
Of the latter no instance can be pointed out, because the process 
of shelf-formation is repeatedly (though with intervals of thousands 
of years, we may nevertheless use this word geologically) modified 
by relative movements of land and sea. 
To get a clear insight into the influence of these movements on 
shelf-formation, it will be convenient to apply the term gradation- 
plane to the plane which comprises the combined peneplain and 
shelf. The shelf is the submerged portion of the gradation-plane, 
whereas the peneplain represents its emerged portion. 
Now it is obvious that the mode of development of the shelf depends 
on: 
1. the mode of development of the entire gradation-plane ; 
2. the extent to which the gradation-plane has been submerged ; 
3. the position of the gradation-plane. 
First of all the growth of the shelf keeps pace with that of the 
entire gradation-plane, but besides this it also increases or decreases 
according to a larger or smaller portion of the gradation-plane being 
covered by the sea. Finally the area of the shelf also depends on 
the position of the gradation-plane; in case orogenetic movements 
cause it to shift from its original position (gently sloping towards 
the sea) to another, say, a more inclined one, the depth of the 
water on the shelf will, during these movements, increase seaward 
and the consequence will be that the sediments, which are transported 
from the Jand towards the sea, will become incompetent to fill up 
the entire available space; consequently the newly formed beds will 
not reach the sea-level and very little sediment will be left to build 
up these beds and thus to extend the shelf farther seaward. 
The above-mentioned three conditions lead to the following con- 
clusions : 
1st. Diastrophism will in the first instance, nearly always check 
the outgrowth of the shelf, because it generally steepens the slope of 
the existing surfaces both above and below the sea-level, consequently 
also that of the gradation-plane. Initially it will give rise to 
steep coasts with poorly developed deep-lying shelves or none at 
1) CHAMBERLIN unites denudation of the land and the growth of the shelf into 
one larger process called by him gradation. 
T. C. CHAMBERLIN Diastrophism and the formative processes. Il. Journal of 
Geology XXI. p. 528, 1913. 
