486 A. E. 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. Most of them are not more than 
half a mile away, though in some places they may be 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 
]xxvii—viil.) 
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 
serpule and Vermetus, barnacles, small black mussels, nullipores, 
corallines, and some true incrusting corals, such as Porites astreoides, 
and a few others. Usually the living rim rises from 1 to 2 feet 
above low-tide level, because the serpule 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 serpule, etc., they 
consist of zeolian 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 
