304 



ALEXANDER AGASSIZ 



The mushroom rock is eaten away, or rather pounded 



away, along the 



^^ line A C [Fig. 



h.wm. 6 ]> leaving a 



crescent - shaped 



l.w.m. wall at the level 



A, and the water 



Fig. 6 all falls out at O 



[Kg. 7]. 

 At the foot of a vertical cliff a ledge is frequently 

 formed [Fig. 8], and the surf 

 during low tide pounds over 

 A and cuts away all the dotted 

 part A C y forming a regular 

 barrier reef at the foot of the 

 cliff, with as much as eight 

 feet in one case, a regular 

 slope from A to (7. If this 

 slope is protected by Algse or corallines, it is not eaten 



away, otherwise 

 it may make a 

 pothole inside 

 the wall. Then 

 vertical walls are 

 of all shapes, 

 winding around 

 and twisting, 

 forming all kinds 

 of reentering 

 curves [Fig. 10]. 

 Now before I 

 had seen these 

 Serpulae atolls, I had come to the conclusion that the 



Fig. 7 



H.W.M. 



