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BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



a liigh narrow wall with a deep opening at B between its extremities. 

 Should the surf break through the sides at E or F, or both, we should 

 have curved vertical walls left, apparently built up by Serpulse, in reality 

 walls of aeolian rock which may be dug out as I have suggested, either 

 in the case of Figure 2 or of Figure 4. 



When the pinnacle finally drops off, it will in its turn be attacked by 

 the sea, and go either to form a smooth ledge, to be covered with AlgtB 

 and Serpulae according to tlie depth in which it lies, or may in its turn 

 be attacked in a similar way to the ledge, and changed to an atoll or a 

 crescent-shaped serpuline reef according to the dip of the strata. Before 

 the breaking off" of the pinnacle, Algse and Serpulse have already begun 

 to grow upon the flat part of the ledge, and protect it to a great extent 

 from the action of the sea. When the sea no longer washes round the 

 pinnacle, but breaks on the ledge at low water, and finds a part which is 

 not protected by Algag or otherwise, it begins to erode it, the sand formed 

 acting like a churn, and thus little by little forming a deep hole in the 

 centre of the mushroom rock. 



)00 



Fig. 6. 



In the case of a long and wide ledge, there are formed upon it a num- 

 ber of secondary pits and atolls, or pot-holes, as indicated by the heavy 

 lines of Figure 6. Let the walls of these break through and connect 

 adjoining pot-holes and we obtain a vertical wall, of irregular outline, 

 such as is indicated by the dotted line of the figure, which is a diagram 

 of one of the ledges of the outer reef off' the south shore. The wash of 

 the sea may break through the continuous wall, leaving only discon- 

 nected parts standing, or we may have the outer walls on the edge of 

 the ledge left, forming a long trough. 



A flat ledge projecting from the base of a shore cliff", if eaten into in 

 the same way by the surf (Fig. 7), may be worn into a circular reef 

 witli vertical walls, of which the top is protected by Algfe and Corallines 

 or SerpuljE, with a pot-hole near its outer wall, in this case eight feet deep. 

 We might call this a diminutive barrier reef. If the walls are parts of 

 diminutive barrier reefs, the shore cliff" behind them may disappear, and 



