ON THE DESTRUCTION OF ROCKS. 249 



of this is required than that which is open to in- 

 spection on the shores of our own island. In the 

 Western Islands of Scotland, it is often rendered 

 very conspicuous by the prolonged basements and 

 ledges which skirt the cliffs, rising precisely to the 

 level of high water. This effect is exceedingly re- 

 markable in Sky, and in the neighbouring islands ; 

 where it is not limited to any class of rock, although 

 most conspicuously exhibited in the trap family. 

 Thus we find towers and pinnacles rising out of the 

 sea, pitched on a wide base below it, and waiting for 

 the day which is to level alike the whole structure. 

 Hence the approach to these rugged and dangerous 

 shores is rendered impracticable by the long ledges of 

 rock which skirt them, on which a formidable surf is 

 for ever breaking. Hence also those sunk rocks, the 

 terror of the mariner, and the remains of those which 

 once towered above the water, but are now at length 

 secured from further destruction. Thus, as the at- 

 mosphere destroys, the sea protects, for a time at 

 least, from further injury ; preserving, in these mo- 

 numents, the most impressive records of what the 

 land once was. In Arran, the long piers, the re- 

 mains of trap veins, which stretch out into the sea on 

 all sides, thus serve to mark the former extent of the 

 less durable land which has at length submitted to 

 that power from which even these records will not 

 for ever be exempted. 



It is not however to frost alone that we must at- 

 tribute those enormous masses of ruin which so often 

 fall in alpine regions, burying the plains together 

 with their inhabitants, and, in our own diminutive 

 Alps, causing those slides of large portions of the 

 hills which may be seen in numerous places. In 

 these slides, -the geologist may often study the dimi- 



