AGASS1Z : BERMUDAS. 267 



thus leave a wall standing apparently isolated from the level of the 

 ledge. It is not surprising that a cursory examination of these walls and 

 atolls should have induced the earlier observers to attribute the growth 

 to Serpulae. 



Rice accepts the theory that the serpuliue reefs are duo alone to the 

 upward growth of Serpuke. He has also observed the circular ridges of 

 coral on the outer ledges (Millepores) similar to the serpuline reefs, 

 except that they are less elevated, their upward growth being limited 

 by the inability of the corals to survive an exposure above the water. 

 These circular reefs 

 are either serpuline or 

 edged by Millepores, 

 and are called boilers ; 

 off the south shore 

 they are generally ser- 

 puline, with only here 

 and there a Millepore 

 boiler on the inner 

 patches. Along the 

 east shore of Harring- 

 ton Sound there are 

 a number of such Mil- 

 lepore atolls and barrier reefs which are merely rims of pot-holes pro- 

 tected by a growth of Millepores. A number of these Millepore reefs 

 extend eastward on the northern side of the main channel off Spanish 

 Point. Along the shores of Harrington Sound we meet occasionally one 

 of the vertical walls or irregularly shaped pot-holes of a projecting ledge, 

 the top of which is protected by a gi-owth of Millepores, much as the Ser- 

 pulae and Algse protect the walls of the pot-holes off the south shore. On 

 many of the patches to the north of the main channel, in a line from 

 Ireland Island to St. George, we find a similar growth of Millepores pro- 

 tecting the exposed ridges of aeolian rocks. Thomson also considers the 

 serpuline reefs as due to the agency of Serpula?. 



The existence of such a variety of reef forms under the same conditions 

 naturally suggests whether the explanation by mechanical causes, such 

 as finds here its application on a small scale for serpuline reefs, may 

 not be a natural explanation also of the formation of the complicated 

 systems of atolls of the Carolines, Marquesas, and Maldives, and whether 

 small simple atolls like the typical ones of the Pacific, and many with 

 complicated outlines, may not in many cases be due to the scouring ac- 



Fig. 7. 



