14G 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 arc 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. 



