The mere extent of the land surface does not, within limits,
affect the question of the rate of denudation. This arises from
the fact that the rain supply is quite insufficient to denude the
whole existing land surface. About 30 per cent. of it does not,
in fact, drain to the

[1] See especially Geikie's Address to Sect. C., Brit. Assoc.
Rep., 1399.

15

ocean. If the continents become invaded by a great transgression
of the ocean, this "rainless" area diminishes: and the denuded
area advances inwards without diminution. If the ocean recedes
from the present strand lines, the "rainless" area advances
outwards, but, the rain supply being sensibly constant, no change
in the river supply of salts is to be expected.

Age-long submergence of the entire land, or of any very large
proportion of what now exists, is negatived by the continuous
sequence of vast areas of sediment in every geologic age from the
earliest times. Now sediment-receiving areas always are but a
small fraction of those exposed areas whence the sediments are
supplied.[1] Hence in the continuous records of the sediments we
have assurance of the continuous exposure of the continents above
the ocean surface. The doctrine of the permanency of the
continents has in its main features been accepted by the most
eminent authorities. As to the actual amount of land which was
exposed during past times to denudative effects, no data exist to
show it was very different from what is now exposed. It has been