146 
NEWER PLIOCENE PERIOD. 
[Ch. XI. 
Chesil bank, or to materials cast up by a wave of the sea upon 
the land, or those which a submarine current has left in its 
track. The kind last mentioned must necessarily, when the 
bed of the ocean has been laid dry, resemble terrestrial allu- 
viums, with this difference, that if any fragments of organic 
bodies have escaped destruction they will belong to marine 
species. 
During the gradual rise of a large area, first from beneath 
the waters, and then to a great height above them, several 
kinds of superficial gravel must be formed and transported 
from one place to another. When the first islets begin to 
appear, and the breakers are foaming upon the new-raised 
reefs, many rocky fragments are torn off and rolled along the 
bottom of the sea. 
Let the reader recall to mind the action of the tides and 
currents off the coast of Shetland, described in the first volume*, 
where blocks of granite, gneiss, porphyry, and serpentine, of 
enormous dimensions, are continually detached from wasting 
cliffs during storms, and carried in a few hours to a distance 
of many hundred yards from the parent rocks. Suppose the 
floor of the ocean not far from the coast to be composed of 
those secondary strata of which several islands of this group 
consist. Such a tract, after being strewed over with detached 
blocks and pebbles of ancient rocks, might be converted into 
land, and the geologist might then, perhaps, search in vain 
for the islands whence the fragments were originally derived. 
For the islands may have wholly disappeared, having been 
gradually consumed by the waves of the ocean, or submerged 
by subterranean movements. 
Let us farther suppose this new land to be uplifted during 
successive convulsions to the height of 1000 feet. The marine 
alluvium before alluded to would be carried upwards on the 
summits of the hills and on the surface of elevated platforms. 
It might still constitute the general covering of the country, 
being wanting only in such valleys and ravines as may have 
* Chapter xv. 
