270 T. C. CHAMBERLIN 



slopes of various moderate degrees as well. Sediments that come 

 into place only by sliding or rolling usually require a declining gradi- 

 ent. It is admissible to regard the abysmal beds, looked at broadly, 

 as essentially horizontal. Additions to them are made by sheets of 

 flotation sediment falHng from above with some approximation to 

 uniformity. The thicknesses that may be attained by such beds 

 in the ocean, so long as the crust maintains a static condition, are 

 strictly limited by the depth of the ocean. This seems a mere 

 truism and falls in with the instinctive inferences that have given 

 rise to the common conviction. 



It is, however, admissible, if not imperative, to regard the sea- 

 shelves as sloping from the continents oceanward, and to recognize 

 that in the very nature of their formation, they persist in sloping 

 systematically toward the ocean at all stages of their formation when 

 they are built up normally. The slopes on the present continental 

 shelves vary through a considerable range. The precise slope is 

 not a matter of special moment in this paper, though it is important 

 in special problems. It will serve our present purpose to recognize 

 slopes as being often as low as three feet per mile and often as high 

 as twelve feet per mile, or from i in 1,760 to i in 440, while both 

 lower and higher slopes are not uncommonly observed. For this 

 discussion let us take the conservative figure of four feet per mile, 

 or I in 1,320, as representative. While it is not here important to 

 assume one approximate figure rather than another, it is important 

 to recognize that some such slope is a systematic feature of the 

 shelf-sea deposits in their normal state, that it is inherent in the 

 nature of the case, and that it may safely be presumed to have 

 affected the sea-shelf deposits of all periods. 



At the oceanward edge of these slightly sloping shelves, the 

 topset beds join at an angle or curve the foreset beds that form the 

 steeper abysmal slope. While the dips of the foreset beds have 

 large variations, they are usually much higher than the topset beds 

 and this higher dip is a systematic feature. Such higher dips must 

 be presumed to have affected the foreset beds in all stages of shelf 

 growth and to be a persistent feature. Willis generalizes the com- 

 moner angles of the present abysmal slopes at 2° to 5°, which may 

 be translated, roundly, into i in 30 to i in 12, but much higher and 



