268 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



tion of the sea working on a larger scale upon substructures easily yield- 

 ing to its effects. When examining'the Hogsty Atoll of the Bahamas I 

 could not fail to be struck by the overpowering mechanical force inces- 

 santly at work. Huge breakers were constantly pouring an immense 

 volume of water over the windward sea face of the atoll, filling its cup to 

 overflowing, and it could find no outlet except over those parts of the 

 sides which were lower than the windward face, or through which, near 

 the lee ends, passages of considerable depth had been eroded, or, finally, 

 through the still deeper channel between the lee extremities of the reefs 

 forming the entrance to the atoll. That is, given a bank of suitable depth 

 upon which corals can flourish, and upon a belt of a certain width, they 

 will form a protective coating to the underlying rocks, just as the serpu- 

 line growths protect the rims of their diminutive atolls. Corals will 

 naturally, from the centrifugal action of the sea, grow on the outer faces, 

 and most abundantly in the direction in which they find least resist- 

 ance in the way of detritus and other accumulations. The sea breaking 

 over them will excavate a lagoon, and break through the sides or lee 

 face to allow the water to flow out through the points of least resistance, 

 and through the entrances to the lagoon. All these causes are impor- 

 tant factors in any theory of coral reefs, and show the complexity of the 

 problem, and the impossibility of framing a single hypothesis to explain 

 the formation of coral reefs in all parts of the world. 



Before seeing the serpuline atolls of the Bermudas it had occurred to 

 me that the configuration of the Hogsty Reef and the formation of its 

 atoll might be due to mechanical causes. We may imagine a bank ol 

 the proper depth, whether formed during subsidence or elevation is im- 

 material, on which corals begin to grow and form a barrier to the surf. 

 The breaking of the surf over this living and protecting barrier digs out 

 the least resisting portions of the surface of the bank, and the material 

 thus dug out finds its way out on the opposite side. Little by little a 

 lee channel is thus formed by the scouring of the mass of water poured 

 over the reef into the incipient lagoon, and a lagoon may be formed on a 

 large scale in the manner described for the formation of the serpuline 

 lagoons of the Bermudas. We may imagine the Hogsty Reef at one time 

 to have been a bank formed by a series of small, low a3olian hills, which 

 have been worn away and have disappeared from the same causes which 

 acted on a larger scale at the Bahamas. The Hogsty Bank was thus 

 brought by subsidence and erosion to its present level, or nearly so, 

 after the growth of a barrier of reefs on the remnants of the teolian hill 

 ledges, then began the action of the surf in eating away the central part 



