HOW AEE ROCK-POOLS MADE ? 197 



which defies the action of the elements for an undefin - 

 able period ; for it seems liable to little change. It is 

 probably comparatively unalterable, or alterable slowly, 

 beneath the level of the lowest tide. But between tide- 

 marks, the perpetual change from wetness to dryness 

 and back again, and the incessant wash of the waves, 

 which frequently beat and dash upon the eroded sur- 

 face with immense violence, are continually grinding 

 down the projecting points and thin walls of stone, and 

 thus creating a new surface, to be bored afresh by new 

 generations of Mollusca. 



It has seemed to me that these burrows have played 

 and are playing an important part in the formation of 

 the numberless rocky basins which we call tide-pools, 

 and in which we marine naturalists so much delight. 

 Let us look at the process. About half-tide level there 

 is a mass of bored rock, from whose burrows the tenants 

 are dying out for want of sufficiently long water- covering. 

 A heavy sea is breaking over it, which has snapped off 

 the thin partition beneath two contiguous burrows, 

 breaking it into several sharply angular bits, which fall 

 into the hole. The whirling and eddying of the waves 

 rattle and roll these fragments round and round day 

 after day, week after week, till at last they are ground 

 to nothing: but an equal effect has been produced 

 on the hollow which held them ; its cavity has been 



