258 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



same condition of things, — an outer ledge of boilers together with irregu- 

 lar inner rows of ledges running close to the line of low-water mark, and 

 gradually passing into the mushroom-shaped ledges which still form a 

 part of the shore cliffs. 



The district extending from Sinky Bay west and east is specially in- 

 structive, as showing the method of irruption of the sea through low 

 shore cliffs to form small boat bays, and the gradual passage of these 

 shore cliffs to lines of rocks and islets running parallel to the coast, and 

 cutting out such bays as Whale Bay, Bailey Bay, Warwick Bay, Great 

 Turtle Bay, etc. We may next follow the passage of these cliffs to 

 submerged ledges, and their transformation into the boilers off the 

 south shore and the outer line of boilei's forming the so called reef 

 off the south shore. The most striking of the serpuline reefs are the 

 fringing and barrier reefs, and their outlying atolls, off one of the points 

 at the east end of Whale Bay, together with the lines of atolls and 

 variously shaped serpuline reefs extending to the eastward. Nowhere 

 perhaps on the south shore do we see so clearly the transition of the 

 isolated mushroom rock ledges surmounted by seolian pinnacles into the 

 ledges which are to become serpuline reefs, as in the district between 

 Great Turtle Bay and the bay at the foot of Gibbs Lighthouse. 



From the descriptions given above of such a variety of reefs formed 

 by the serpuline ledges, and of the action of the sea upon them, we 

 may obtain on a small scale an illustration of the mechanical theory 

 of the formation of some coral reefs. This may be specially applicable 

 to the formation of compound atolls, as has already been suggested. 

 We find off the south shore, in the same area and subject to identical 

 conditions, patches which assume the shape of atolls of fringing or bar- 

 rier reefs all within a stone's throw of one another. But in this case 

 the structure of the foundation gives us the explanation of their forma- 

 tion, for the shape of these diminutive reefs is primarily determined by 

 that of the ledge, and not by the growth of the Serpulae. The different 

 shapes of these diminutive reefs can be traced to the manner in which 

 the sea has acted upon their seolian substructure. It may have only 

 honeycombed the surface of a large cliff fragment, and left it as it fell, 

 merely covering its diminutive spires and hollows with a thin layer of 

 Serpulae, Mytilus, Alga?, and Corallines. It may have washed off from the 

 shore cliffs slabs of seolian rock in such a manner that as they lie on the 

 beach the strata are horizontal, and, the edges having become cemented 

 by the action of the sea, the division lines become obliterated, and over 

 their surface has grown an animal and vegetable covering. It is not 



