156 C. A. COTTON 
The effects of fluctuations in the rate of subsidence of the sea 
floor and in the rate of supply of waste may be considered together. 
When the ratio of waste supply to rate of subsidence is sufficiently 
large the front of the shelf will advance (Fig. 6, B); and when this 
ratio is sufficiently low the front will retreat (Fig.'6, C or D). 
Notable fluctuations in the value of the ratio will determine 
alternating periods of advance and retreat, and these alternations 
will produce interstratification of beds of very varied lithological 
character in the shelf deposits which are not without parallel in the 
known rocks. 
In the landward portions of the shelf intercalations of subaérially 
deposited material may be present in the dominantly marine top- 
set beds, as Barrell has shown to be the case in those portions of 
Fic. 6.—Diagram illustrating the growth of a shelf during a period of subsidence. 
A, edge of the shelf with sea-level SL*; B,C, D, possible positions of the edge of the 
shelf with sea-level SL?. 
the shelf which are the submarine portions of deltas.t. An instruc- 
tive example of such alternation has recently been described by 
Stebinger.? 
Farther seaward the mass of sediment deposited during sub- 
sidence may be for a considerable distance composed of marine 
topset material throughout. The texture of these topset beds will 
depend very largely upon the relief and also upon the nature of the 
rocks of the neighboring land; and it may be expected to vary ver- 
tically, fine sediments marking periods when the land has been 
reduced to low relief by erosion and coarser sediment marking 
periods of renewed differential elevation. The topset material will 
tJ. Barrell, ‘Relative Geological Importance of Continental, Literal and Marine 
Sedimentation,” Jour. Geol., XIV (1906), 353-54, Fig. 10, p. 445; ‘‘Criteria for the 
Recognition of Ancient Delta Deposits,” Bull. Geol. Soc. Am., XXIII (1912), 309, 
Fig. 4. 
2. Stebinger, ‘The Montana Group of Northwestern Montana,” U.S. Geol. 
Surv., Prof. Paper g0G, 1914, pp. 61-68. 
