154 CA. COTTON 
throughout which there has been a steady stream of sediment from 
the land. If now this already formed shelf be supposed to subside 
very slowly, as the shelf sinks, the stream of waste supplied by 
rivers from the adjacent land may be reduced in volume, may 
remain sensibly constant, or may be increased in volume, according 
as the land subsides also, remains stationary, or is uplifted. Let 
us suppose, however, that the stream of waste is sufficiently con- 
tinuous and the movement sufficiently slow to allow the shelf to 
be maintained during subsidence. Obviously maintenance on a 
sinking floor of a shelf with its outer edge at a fixed depth (approxi- 
mately 100 fathoms) below sea-level involves deposition of material 
on the top of the shelf as well as, or instead of, in front of it. Less 
of the waste will be available for deposition as foreset beds than 
in the case of a shelf being built during a period of stillstand. The 
shelf will be built upward of material arrested in transit seaward | 
owing. to its being lowered below the depth at which wave action 
stirs it sufficiently to keep it in motion. 
This accumulation of topset beds may attain an enormous 
thickness, as has been shown by Barrell to be the case in deltas, the 
thickness depending only upon the amount of subsidence during 
which a continuous supply of waste is kept up. 
Assuming that during subsidence the supply of waste is always 
sufficiently abundant to insure the continuity of deposition of top- 
set beds over at least a portion of the area, we may investigate the 
conditions of deposition of topset beds, foreset beds, and pelagic 
deposits, understanding by the latter term deposits formed either 
in moderatly deep or relatively shallow water where the supply of 
bottom waste fails, the deposit consisting of organic and minute 
inorganic particles settling from suspension (‘‘flotation beds,” 
Chamberlin). 
The form of a single stratum deposited in a given time during 
stillstand will be that shown in Fig. 5a, that is to say, the bulk of 
the terrigenous sediment will be in the foreset portion, the edge of 
the shelf advancing seaward a considerable distance during the 
given time. With continuous subsidence in progress, however, the 
formation of thick topset beds may use up so much of the waste 
that there is little or none left to build the edge of the shelf forward. 
