524 T. C. CHAMBERLIN 
ceasing only when its systematic work is cut off by renewed diastro- 
phism. If diastrophism is periodic, effective gradational results 
are inter-periodic products; diastrophism and systematic gradation 
are alternately dominant. From these general considerations, let 
us turn to specific features. 
The distinctive characteristics of shelf-seas are these: 
t. There is a close approach to parallelism between the surfaces and 
the bottoms of these seas. There is but a gentle slope between their 
landward sides and their seaward sides or their deepest axes. In 
their immature stages the typical slopes may be I in 1,000 or I in 
Soo or steeper; in their maturity the slopes seem often to have fallen 
to I in 2,000 or 3,000 or lower. This close approach to parallelism 
between surface and bottom is a feature of moment in its bearings 
on the nature of the deposits and the character of the life. 
2. The parallelism between sea-surface and sea-bottom is close in 
the further sense that the two planes lie near one another. For reasons 
inherent in the gradational processes, the sea-shelf is limited in 
depth. Beyond a certain depth of water effective transportation of 
sediment fails, and the further growth of the shelf is checked except 
as the deep in front is filled. For the limital depth, let us assume 
600 feet, 200 meters, 100 fathoms, the recognized mean depth of the 
outer edge of the present continental shelf. Let considerable 
variations from this be recognized as consistent with the type, but 
this figure may serve as representative. Exactness in this particu- 
lar is not material to the purposes of this discussion, for the dis- 
tinctions to be drawn are so broad that great latitude is permissible 
without invalidating the arguments built upon them. The natural 
criterion of the type is that depth at which the agitation of waves, 
tides, and currents ceases to keep in effective suspension or in rolling 
condition so much of the terrigenous silts as constitute the larger 
mass of the earthy matter derived from the land. The extremely 
fine material that may float long and far with little motion is 
negligible as it does not build up the bottom on which it falls apace 
with the more bulky constituents. The shelf is thus a product 
whose configuration is determined by its own conditions; it is a self- 
regulated formation; the guiding element in whose genesis is the 
sea-surface and the agencies that play upon it. 
