A. FE. Verrill—The Bermuda Islands; Geology. 121 
he mentioned, however, happened to be in shore rocks, within reach 
of the tides at least. However, those that I have figured (pls. xix, 
xx) are decidedly above the tides, and if they were ever worn by 
the waves, it must have been in a period of greater subsidence, the 
Figure 26.—Diagram of group of small pot-holes on edge of reef; a, the dotted 
line, shows how some may become crescent-shaped by erosion, as ¢, e. 
Altered from A. Agassiz. 
existence of which Mr. Agassiz does not admit. But they have 
sharp edges; are surrounded by still adherent red-clay soil; their 
inner surfaces are nearly smooth, and they cut uniformly through 
the harder and softer layers, which are characters not found in real 
pot-holes. Moreover, just the same structures occur in limestones, 
apparently of the same age, at least 60 to 80 feet above the sea. 
Therefore they can hardly be pot-holes, and those that are in the 
sea must, at any rate, have preéxisted in the limestones before the 
present submergence of the rocks. See chapter 24), for a discus- 
sion of the mode of origin of these structures. 
i. Serpuline Atolls or “ Boilers.” 
Along nearly the whole southern shore of the islands the reefs are 
situated much nearer to the shore than on the northern and western 
sides. Most of them are not more than half a mile to a mile away, 
though in some places they may be more than a mile from the shore. 
Along this coast most of the outer reefs are usually flat on the top 
and well covered with living corals, sea-fans and other gorgoniz, 
mussels, barnacles, serpule, and sea-weeds. A few of them, as 
Southwest Breaker, are uncovered in places at low tides. Their 
sides are steep, often perpendicular, and frequently undercut. They 
often fall off into deep water by flat steps or benches of hard lime- 
stone. They seem to be formed, in most places, of the hard, nearly 
horizontal beds of the Walsingham limestone (see above, pp. 73, 74). 
The inner line of reefs that exists along most of this coast is pecu- 
liar in being made up largely of a special form of reefs, usually 
known as ‘‘serpuline atolls,” a name given by Lieut. Nelson in 1840.* 
* See pl. xxiii; also these Trans., xi, p. 486, pls. Ixxvii, Ixxviii; ‘‘ The Ber- 
muda Islands,” p. 74, same plates. 
