254 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Serpulse, Algae, Corallines, and other growths have only protected the 

 surface of the mushroom-shaped seolian rock ledges which form these 

 structures from the action of the breakers. They have not built up the 

 raised rims of the atolls, or the crescent-shaped or the horseshoe-shaped 

 reefs, or the vertical walls forming the irregular convolutions and curves 

 of the broader ledges. 



Before my visit to the Bermudas I accepted the explanation given by 

 older writers of the mode of formation of the atolls, as due to the 

 accelerated growth of Serpulte on their outer rim. I was therefore 

 greatly surprised, on hammering at some of these structures, to find 

 that the vertical walls were not built up, as is generally believed, of ser- 

 puline limestone, but were composed of seolian rock, and to discover that 

 in many cases the elevated rim was protected by the hard ringing crust 

 so characteristic of limestones exposed to the action of the sea, and fur- 

 ther to find that the coating of Serpulie, of Algse, of Corallines, and of 

 Nullipores was quite superficial. 



Some of the serpuline atolls are circular and quite regular in outline, 

 others crescent-shaped, while others are apparently formed by the ac- 

 crescence of two or three atolls. Some of the circular atolls are sym- 

 metrical, with a central depression, at the bottom of which more or less 

 sand has gathered. The rim of these atolls may project from a few 

 inches to one and a half feet, or even more, from the nearly vertical base ; 

 its surface is completely covered by a thick growth of different species 

 of Algse, Zoanthida', Corallines, and Serpulfe. The rim varies greatly in 

 width ; in some cases it is not more than eight to ten inches, in others 

 from one to five feet, and in some cases there is only a small circular 

 pot-hole or a very circumscribed area left bare of growth in the centre. 

 The rim is often greatly developed on the weather side, forming a 

 crescent, tapering gradually to a thin wall on the opposite side. The 

 crescent is often open for a great part of the circumference, the weak wall 

 of rock forming its lee edge having been carried away by the breakers. 



On the outer reef the ledges which are awash are similarly constructed. 

 It is true there are few of the regular atoll shape, by far the greater 

 number being long ledges of compoinid atolls made up of diminutive 

 crescent-shaped reefs. Upon these leilges low vertical walls have been 

 cut out varying from six to eighteen inches in height, following all sorts 

 of curves, rising like a succession of S-shaped loops of circular or cres- 

 cent shape, or re-entering curves, running in all possible ways, and which 

 at first sight would appear to be all due to the growth of vegetable and 

 animal life which covers the top and sides of the walls. 



