

Parr IL. Szcr. ii. § 7.] DENUDATION AND DEPOSITION. 451 
the surface, a submarine plain should be formed along the margin of 
the land. This final result of denudation has been achieved again 
and again in the geological past, as is shown by the existence of 
tablelands of erosion (ante, p. 40). To these tablelands the name of 
“plains of marine denudation” has been applied by A. C. Ramsay. 
From what has now been said, however, it. will be seen that in their 
actual production the sea has really had less to do than the meteoric 
agents. A “plain of marine denudation ”’ is that sea-level to which 
a mass of land has been reduced mainly by the subaerial forces; the 
line below which further degradation became impossible, because 
the land was thereafter protected by being covered by the sea. 
Undoubtedly the last touches in the long process of sculpturing 
were given by marine waves and currents, and the surface of the plain, 
save where it has subsided, may correspond generally with the lower 
limit of wave-action. Nevertheless, in the past history of our planet 
the influence of the ocean has probably been far more conservative 
than destructive. Beneath the reach of the waves the surface of the 
abraded land has escaped the demolition which sooner or later 
overtakes all that rises above them. 
5. Deposition—the framework of new land. 
If a survey of the geological changes in daily progress upon the 
surface of the earth leads us to realise how momentously the land 
is being worn down by the various epigene agents, it ought also to 
impress us with the vast scale on which new formations—the founda- 
tion of future land—are being continually accumulated. Every 
foot of rock removed from the surface of a country is represented 
by a corresponding amount of sedimentary material arranged some- 
where beneath the sea. Denudation and deposition are synchronous 
and co-equal. 
On land vast accumulations of detrital formations are now in 
progress, Alluvial plains of every size, from those of mere 
brooks up to those of the largest rivers, are built up of gravel, sand, 
and mud derived from the disintegration of higher ground. From | 
the level of the present streams successive terraces of these for- 
mations can be followed up to heights of several. hundred feet. 
Over wide regions the daily changes of temperature and wind supply 
a continual dust, which, in the course of centuries, has accumulated 
to a depth of sometimes 1500 feet, and covers thousands of square 
miles of the surface of the continents. ‘The numerous lakes that dot 
the surface of the land serve as receptacles in which a ceaseless 
deposition of sediment takes place. Already an unknown number 
of once existent lakes has been entirely filled up with detrital 
accumulations, and every: stage towards extinction may be traced in 
those that remain. 
But extensive though the terrestrial sedimentary deposits may 
be, they can be regarded merely as temporary accumulations of the 
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