124 A, E. Verrill—The Bermuda Islands; Geology. 
Figure 27 represents a cliff or ledge (d@) eroded at base into a flat 
bench, on the borders of which the serpuline rim (a) has already 
begun to grow, while the waves have scooped out a deep pool or 
cup (e, ¢), which, in this case, is 8 feet deep at the center. This 
would form a fringing atoll. 
Figure 28 represents one of the detached pinnacles of xolian lime- 
stone, in nearly flat beds, at a stage when it has become strongly 
undercut at the base, while the rim of the serpuline atoll (a, @’) is 
already growing and the central pool (s, s’) is being excavated by 
the waves. Such a pinnacle would eventualiy be overthrown, and 
then the atoll would be more deeply excavated near the middle, 
thus assuming the typical form, which is shown in section in figs. 
11, s, and 29. 
Seale=ht feck fz yinch 
Figure 29.—Diagrammatic section of completed, small serpuline atoll; a, a’, rim 
of living organisms; c, central cup, 4 feet deep; s, accumulation of sand 
and gravel ; d, d', undercut submerged xolian limestones ; lw, low-tide level. 
Altered from A. Agassiz. 
The submerged sides (d, d’) of such structures are also being 
eroded by the dash of the waves, especially at low tide and in stormy 
weather, so that most of them are undercut or perpendicular below 
the protecting rim of organisms. Thus, in course of time, many of 
them will be broken off on one side, forming crescents, or all around, 
and thus they will eventually be changed into ordinary submerged 
ledges or reefs. Many such isolated reefs, without the rims, occar 
all along this coast, often mixed with the atolls. 
Similar serpuline atolls occur at a few places and in small numbers 
inside the great bordering reefs of the north side. Some of these 
are to the northeast of Ireland Island and north of St. George’s 
Island. Perhaps they occur there because the character and hori- 
zontal position of the limestone are favorable. 
