486 A. M Verrill — The Bermuda Islands. 



13. — Serpuline Atolls or " Boilers.'''' 



Along nearly the whole southern shore of the islands the reefs are 

 situated much nearer to the shore. -Must of them arc not more than 

 half a mile away, though in some places they may he nearly a mile 

 from the shore. Along this coast most of them have taken on a 

 peculiar form known as " boilers " or serpuline atolls. (See plates 

 lxxvii-viii.) 



These are detached, rounded, elliptical, or irregular reefs with a 

 raised rim and excavated or cup-shaped central part. They vary in 

 size from those only a few feet in diameter up to those of 100 feet 

 or more. Many are very regularly rounded. The rim is formed by 

 a solid, raised, living crust, made up of the hard convoluted tubes of 

 serpula? and Vermetus, barnacles, small black mussels, nullipores, 

 corallines, and some true incrusting corals, such as Porites astri aides, 

 and a few others. Usually the living rim rises from 1 to 2 feet 

 above low-tide level, because the serpula? and mussels, of which it 

 largely consists, can endure an exposure to the air of an hour or 

 two, without inconvenience. But they soon reach their limit of 

 endurance in this respect, and stop growing upward. (See Geology.) 



The seas, even in moderate weather, always break on such reefs, 

 forming a line of outer white breakers nearly parallel with the shore. 



There is also, in many places, as near Hungry Bay, an inner line 

 of these "boilers" of the same structure and form, very near the 

 shore, and sometimes even united to the shore ledges at some points. 



These "boilers" are fundamentally of the same structure as the 

 other reefs, for beneath the marginal crust of serpuhe, etc., they 

 consist of reolian limestone, like all the rest. 



Their hollow or cup-shaped form is due to the heavy seas that 

 dash against the hard outer rim and fall over into the unprotected 

 central area like a cataract, rapidly wearing off and carrying away 

 the soft rock. 



Reefs having this character, in so perfect a form, have not been 

 observed in any other part of the world. 



14. — Channels or Natural Cuts through the Reefs. 



There are, besides the main ship-channel or "narrows," several 

 other channels or "cuts" through the outer reefs on the eastern, 

 northern, and western sides, through which vessels of small size can 

 reach the anchorages and harbors, if they have a good local pilot 

 and favorable winds. Some of these were formerly considerably 



