158 C. A. COTTON ee 
overlying mud layer have been so deeply buried that they are 
secure from further disturbance. The sand of a superficial layer 
is, however, again washed clean, and so the process goes on until 
there are innumerable alternating sand and mud layers throughout 
a great thickness of strata. 
Toward the outer part of the mass of sediment intercalations of 
foreset beds and pelagic deposits may be present in the topset beds 
owing to advances and retreats of the edge of the shelf correspond- 
ing to fluctuations in the ratio of waste supply to rate of subsidence, 
the possibility of which has been pointed out on an earlier page. 
In Fig. 7, which represents diagrammatically a section of a shelf 
Fic. 7.—Alternation of lithological types in shelf deposits resulting from fluctua- 
tion in the ratio of supply of waste to rate of subsidence. Topset beds, black; foreset 
beds, white; pelagic beds, with cross-lines. The top of the shelf at successive stages 
is shown by the white lines. The vertical scale is exaggerated about ten times. 
the front of which has alternately retreated and advanced during 
upward growth, A is the front of a shelf built forward during a 
period of stillstand preceding the subsidence, B, D, and F are 
positions of the front after episodes of small ratio of waste supply 
to rate of subsidence, and C, EF, and G are positions of the front 
after episodes during which this ratio has had a large value. In 
order to simplify the diagram the lateral transitions from one type 
of sediment and from one slope to another are represented as per- 
fectly sharp, but it must be borne in mind that in nature these 
transitions are gradual. 
An inspection of this diagram makes it clear that above A there 
will be in the region represented by the middle part of the diagram 
the following succession: 
