Ch. vi] inland sea- cliffs. 71 



may have proceeded so far, that no ruins are left of the dilapidated 

 rocks. 



Although denudation has had a levelling influence on some countries 

 of shattered and disturbed strata (see fig. 87, p. 63, and fig. 91, p. 69), 

 it has more commonly been the cause of superficial inequalities, espe- 

 cially in regions of horizontal stratification. The general outline of these 

 regions is that of flat and level platforms, interrupted by valleys often of 

 considerable depth, and ramifying in various directions. These hollows 

 may once have formed bays and channels between islands, and the 

 steepest slope on the sides of each valley may have been a sea-chfl", which 

 was undermined for ages, as the land emerged gradually from the deep. 

 We may suppose the position and course of each valley to have been 

 originally determined by diff"erences in the hardness of the rocks, and by 

 rents and joints which usually occur even in horizontal strata. In mountain 

 chains, such as the Jura before described (see fig. 71, p. 55), we perceive 

 at once that the principal valleys have not been due to aqueous excava- 

 tion, but to those mechanical movements which have bent the rocks into 

 their present form. Yet even in the Jura there are many valleys, such 

 as C (fig. 71), which have been hollowed out by water ; and it may be 

 stated that in every part of the globe the unevenness of the surface of 

 the land has been due to the combined influence of subterranean move- 

 ments and denudation. 



I may now recapitulate a few of the conclusions to which we have ar- 

 rived : first, all the mechanical strata have been accumulated gradually, 

 and the concomitant denudation has been no less gradual : secondly, the 

 dry land consists in great part of strata formed originally at the bottom 

 of the sea, and has been made to emerge and attain its present height 

 by a force acting from beneath : thirdly, no combination of causes has 

 yet been conceived so capable of producing extensive and gradual denu- 

 dation, as the action of the waves and currents of the ocean upon land 

 slowly rising out of the deep. 



Now, if we adopt these conclusions, we shall naturally be led to look 

 everywhere for marks of the former residence of the sea upon the land, 

 especially near the coasts from which the last retreat of the waters took 

 place, and it will be found that such signs are not wanting. 



I shall have occasion to speak of ancient sea-clifis, now far inland, in 

 the southeast of England, when treating in Chapter XIX. of the denu- 

 dation of the chalk in Surrey, Kent, and Sussex. Lines of upraised 

 sea-beaches of more modern date are traced, at various levels from 20 to 

 100 feet and upwards above the present sea-level, for great distances on 

 the east and west coasts of Scotland, as well as in Devonshire, and othei 

 counties in England. These ancient beach-lines often form terraces of 

 sand and gravel, including littoral shells, some broken, others entire, and 

 corresponding with species now living on the adjoining coast. But it 

 would be unreasonable to expect to meet everywhere with the signs of 

 ancient shores, since no geologist can have failed to observe how soon all 

 recent marks of the kind above alluded to are obscured or entirely ef- 



