122 A, E. Verrill— The Bermuda Islands; Geology. 
The serpuline atolls are detached, rounded, elliptical, crescent- 
shaped, 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 regu- 
larly rounded. The rim is formed by a solid, raised, living crust, 
made up of the hard, convoluted, shelly tubes of serpule and Verme- 
tus, barnacles, small black mussels, nullipores, corallines, and some 
true incrusting corals, such as Porites astreoides and a few others, 
with more or less seaweeds, etc. 
The living rim of these atolls is usually laid bare by the ebb tide, 
wholly or in part. The rim is usually higher and larger on the 
windward side, because the organisms live best in the swash of the 
pure water, and are liable to be killed off on the lee side by the sand 
and debris, often washed out from the central pool. The growing 
rim, therefore, is often lacking at one or more places on the lee side, 
so that the edge is lower, and the water that is thrown into the 
central pool by the waves rushes out over the low lee side in a minia- 
ture cataract, when there are large waves. The rim may rise from 
a foot to nearly two feet above low tide, because such organisms as _ 
compose it can endure an exposure to the air of two or three hours, 
especially as the sea or spray usually dashes over them, and they 
retain water in their interstices. (Plate xxiii.) 
The living organisms usually have not built up the whole height 
of the raised rim, but they have protected it from erosion to a lower 
level, and have added something to its height by their own growth. 
These serpuline atolls are composed, like the reefs farther out, of 
hard zolian limestones, usually in nearly horizontal beds, probably 
of Walsingham age (see pp. 73-74). The hardness and horizontal 
position of the beds of this limestone are eminently favorable for 
their formation, though they probably are often formed of other 
limestones, especially when they are in hard and nearly flat layers. 
If the layers happen to be much inclined, the atolls become irregular 
and imperfect, owing to the very uneven erosion that results. 
The submerged sides of the atolls are usually undercut, or at least 
very steep. They are situated at various distances from the shore, 
but are mostly within half a mile of it, and usually with not more 
than 10 to 15 feet of water between. Many are in water not more 
than 2 to 4 feet deep at low tide. In some places many of them are 
even connected with the shore ledges, at low tides, as “fringing 
reefs,” especially around the outer small islets, but in such places 
the rim is covered more by seaweeds, etc., than by serpule. 
