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Sect. VLj 



GEOLOGY 



177 



known of their movement under water. In what con- 

 dition are the pebbles '^—are they encrusted (as often 

 happens) with delicate corallines — after a heavy gale are 

 the spines of such corallines found broken? In narrow 

 channels where there are rapid currents, and in the open 

 sea in front of straits, where the water often suddenly 

 deepens, what is the nature of the bottom ? To what 

 depth does the sea in a storm render the water muddy ? 



far from the beach, and to what depth, does the 



How 



recoil of the waves, or the 



" undertow," act, for 



m- 



stance, on light anchors ? At what depth can the sea wear 

 solid rock ? This may sometimes be judged of by the 

 nature of the bottom ; thus, where soft mud overlies the 

 rocky surface, we may infer that the sea can hardly now be 

 a destroying agent, even if the inclination of the strata 

 on the adjoining coast shows that rocky strata must once 

 (probably, w^hen the land stood at a different level) have 

 extended much further. Is it at the line of hio'h or 



low water, or between them, that the breakers most 

 vigorously eat into coast-cliffs ? Gigantic fragments of 

 rock, much too large to be themselves rolled about, 

 may be seen at the foot of almost every line of high cliffs ; 



by what means in the course of time will these be re- 

 moved, as must have happened v/ith their innumerable 

 predecessors? Are they slowly worn away or broken 

 up? It may be well to recollect tliat in the tropics the 

 powerful action of frost in splitting stones is entirely 

 eliminated. Our observations, moreover, on the alluvial 

 and sub-littoral deposits of these latitudes are not per- 

 plexed by the ancient eli^cts of floating ice. The spray 

 of salt-water, above the line of breakers, corrodes by 



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