riEItCED BOCKS. \^^ ■*«>, 93 



doubled speed upon its bottom, wher€h^B&r/((jl<?]^0>^ 

 debris of the rock, dashed about with irninen§B^fce, 

 helps also to break up the rock and make fresh 

 debris. In this way the depth of the excavation 

 increases, its sides are enlarged, and at last daylight 



Fig. 18. — Hocks worn through by tJie Waves. 



is let in from the other side. To produce this result, 

 however, the rock must not be too thick, because 

 there is obviously a limit m the depth of the exca- 

 vation at \vhi6h the wave loses its force" 



