SLIDES, JRO SIA GLOIEIN TOS. 75 
their vertical range. They cannot extend above the level reached 
by the crests of the highest waves, and in great thickness they 
cannot extend below the depths reached effectively by the bot- 
toms of the waves. Although small amounts of fine sediment 
may be carried far out into lakes or into the sea, especially along 
the courses of currents, yet considerable deposits of lacustrine or 
marine sediments coarser than fine clay must be confined to a 
narrow belt of shallow water along the shores. Even within this 
belt there is a systematic gradation of material on the basis of 
size, the coarser being left nearer the shore, while only the finer 
reaches the outer part of the zone of abundant deposition. 
If the level of the standing water along whose shores depos- 
its are made varies, the shore deposits made at successive stages 
will have a corresponding vertical range. The deposits making 
at any given time must have an essentially horizontal upper limit, 
corresponding approximately with the level of the water. The 
earth’s surface parts are known to be undergoing differential ver- 
tical movements, and are known to have suffered such movements 
in the past. It follows that shore deposits, horizontal in position 
at the time they were made, may not always remain so. But if 
they fail to preserve their original horizontal position, they can 
never assume any new position except such as might be given 
them by crustal movements. 
No lake and no combination of lakes could cover the area 
which the drift covers. The only body of water which could 
have been everywhere where the drift is, is the sea. If the sea 
covered the whole of this area, including its mountains, and if 
the land gradually rose, or the sea receded, all parts of the drift 
area would have found themselves, sooner or later, at the level 
of the sea. In this way only could all parts of the drift-covered 
area have been affected by those parts of the sea along which, 
and along which only, abundant deposition of coarse materials 
can take place, if water alone be concerned. 
To account for the absence of drift from the southeastern part 
of the United States, lower on the whole than the drift-covered 
territory to the north, it would be necessary to suppose that this 
