8] These figures are merely used to give an idea of the rate at which
denudation is actually going on now; but if no elevatory forces were at
work, the rate of denudation would certainly diminish as the mountains were
lowered and the slope of the ground everywhere rendered flatter. This would
follow not only from the diminished power of rain and rivers, but because
the climate would become more uniform, the rainfall probably less, and no
rocky peaks would be left to be fractured and broken up by the action of
frosts. It is certain, however, that no continent has ever remained long
subject to the influences of denudation alone, for, as we have seen in our
sixth chapter, elevation and depression have always been going on in one
part or other of the surface.

[89] The following statement of the depths at which the Palaeozoic
formations have been reached in various localities in and round London was
given by Mr. H. B. Woodward in his address to the Norwich Geological
Society in 1879:--

_Deep Wells through the Tertiary and Cretaceous Formations._

Harwich at 1,022 feet reached Carboniferous Rock.
Kentish Town ,, 1,114 ,, ,, Old Red Sandstone.
Tottenham Court Road ,, 1,064 ,, ,, Devonian.
Blackwall ,, 1,004 ,, ,, Devonian or Old Red Sandstone.
Ware ,, 800 ,, ,, Silurian (Wenlock Shale).

We thus find that over a wide area, extending from London to Ware and
Harwich, the whole of the formations from the Oolite to the Permian are
wanting, the Cretaceous resting on the Carboniferous or older Palaeozoic
rocks; and the same deficiency extends across to Belgium, where the
Tertiary beds are found resting on Carboniferous at a depth of less than
400 feet.

[90] _Geological Magazine_, Vol. VIII., March, 1871.

[91] Mr. C. Lloyd Morgan has well illustrated this point by comparing the
generally tilted-up strata denuded on their edges, to a library in which a
fire had acted on the exposed edges of the books, destroying a great mass
of literature but leaving a portion of each book in its place, which
portion represents the thickness but not the size of the book. (_Geological
Magazine_, 1878, p. 161.)

[92] Professor J. Young thinks it highly probable that--"the Lower
Greensand is contemporaneous with part of the Chalk, so were parts of the
Wealden; nay, even of the Purbeck a portion must have been forming while
the Cretaceous sea was gradually