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430 _ DYNAMICAL GEOLOGY. [Boos II]. 
several miles, the grind of the stones upon each other, as they are 
dragged back by the recoil of the waves which had launched them - 
forward. In this tear and wear the loose stones are ground smaller, 
and acquire the smooth round form so characteristic of a surf-beaten 
beach, At the same time they bruise and wear down cliffs against. 
which they are driven. A rock much jointed, or from any cause 
presenting less resistance to attack, is excavated into gullies, creeks, 
and caves; its harder parts standing out as promontories are pierced ; 
gradually a series of detached buttresses and sea-stacks appears as 
the cliff recedes, and these in turn are wasted until they become 
mere skerries and sunken surf-beaten reefs (Fig. 161). At the same 
time the surface of the beach is ground down. ‘The reality of this 
erosion and consequent lowering of level is sometimes instructively 
displayed where a block of harder rock serves for a time to pro- 



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Fic. 161.—Coast or CornwaLt, at BEDRUTHAN (DEVONIAN ROCKS), CUT BY THE 
Sea Into Cuirrs, Bays, anp Stacks (B.). 
tect the portion of rocky beach lying beneath it. The block by 
degrees comes to rest on a growing pedestal which is eventually cut 
round by the waves, until the overlying mass, losing its support, 
rolls down upon the beach, and the same process is renewed 
(Fig. 162). 
Of the progress of marine erosion the more exposed parts of the 
British coast-line furnish many admirable examples, The west coast 
of Ireland, exposed to the full swell of the Atlantic, is in innumerable 
localities completely undermined by caverns, into which the sea 
enters from both sides. ‘The precipitous coasts of Skye, Sutherland, 
Caithness, Forfar, Kincardine, and Aberdeenshire abound in the most 
impressive lessons of the waste of a rocky sea-margiu ; while the same 
Ee pmesane features are prolonged into the Orkney and Shetland 
slands, the magnificent cliffs of Hoy towering as a vast wall some 
1200 feet above the Atlantic breakers, which are tunnelling and fret- 
ting their base. 
