i 



'^ 



4 



i 



178 



GEOLOGY. 



[Sect. VI. 



chemical decomposition calcareous rocks; does tliis play 

 any important part on other rocks ? Most bold coasts 

 are fronted by sharp promontories and even isolated 



pmnacies } 



arc 



these excl :ively due to tlie greater hard 



i: -ss of the rocks composing them, or do not the breakers 

 act more efficiently when 



t T • 



round 



eaaymg rouna any ^..^ 



slight 



projection r 



Rocks rising steeply out of the open ocean, and cx- 



poo^id to the incessant wash of the heaviest surf, are often 



thicMy coated over with various marine animals, and 



this w^ould seem to indicate that pure water has not the 



power of gradually wearing av/ay hard rocks, though the 



waves may occasionally tear oiF large fragments. Is the 



washing to and fro of pebbles, or of sand, a necessary 



element in the corroJhig power of waves on hard rocks ? 



but how comes it that small land-locked harbours, where 



the waves can hardly have force to move the shingle, 



should ever be surraunded by cliffs, which, in most 



cases, clearly prove that considerable masses of rock 



have been vy-orn down into mud and removed ? Again, 





covered 



shingle^ does the rolling to and fro of the pebbles wear 

 away solid rock ? if so, the pebbles would be clean, and 

 the submarine rocky surface probably w^orn into farrows 



or channels at right andes to the beach. Where there 



are violent currents and eddies, are deep round holes 

 worn in the bottom, like those produced by eddies at the 

 foot of cascades? This, perhaps, might bo ascortaiiied 



polo at the turn of the tide : deep round 



bv a lonpr 



holes have been observed on rocks formerly eo-v 



'cred 



by tl 



1143 sea, and their 



ormm 



h 



■i ■> 



perplexed geologists. 



p 



/ 



se- 



*f 



An 



sh 

 of 



A 



y J 



m 



^v 



la 



d- 



3 



or 



X, - 



li 



irr 



OY 



S" 



be 



ffi] 



rei 



t^ 



. * ^ 



pO] 



fo 



r 



m 



ft 



or 



t/ 



Uj 



r 



/\ 



<f'. 



