92 THE BOTTOM OF THE SEA. 



pierced rocks. Sometimes they are found isolated 

 in mid-ocean ; sometimes they are united to the land, 

 or only separated by narrow, and perhaps tortuous, 

 channels. Let us hear what M. de Tessan has to 

 say, in the work already quoted, of the manner in 

 which waves act upon such rocks : — 



«Lat. 25° 0-9' N.; long. 120° 76' W.— We have 

 passed very near the Alijos Kocks, which are still 

 marked as doubtful in some charts. The highest of 

 them rises about 150 feet above the sea. It is 

 pierced through in the direction from south-east to 

 north-west. This phenomenon of pierced rocks oc- 

 curs most often when the rocks are composed of 

 superimposed beds, of no great thickness, and not 

 well compacted one with another. It is to be ex- 

 plained by the action of the billows ; in fact, their 

 most destructive effect is produced upon the perpen- 

 dicular face of the rock, about the middle of its 

 elevation, and in the direction in which the waves 

 strike against it. The rock of course has a tendency 

 to crumble away at that point more than elsewhere, 

 and if its composition be such that it cannot resist 

 the incessant shock of the waves, an excavation 

 necessarily results. The hollow, once formed, be- 

 comes itself the cause of an increased destructive 

 effect on the action of the waves. Gliding along 

 tlie sides of the excavation, they strike with re- 



