114 A, EF. Verrill— The Bermuda Islands; Geology. 
submerged, so that boats can goin. It has apparently been made 
by the removal of a mass of softer limestone. It may have been a 
passage-way for an underground stream when the land stood at a 
higher level (p. 76). But large masses of stone have fallen from the 
sides and roof in modern times, due to undercutting, and many 
masses still le on the bottom, in plain view. 
There are also small grottoes on Trunk Island, above and below 
tide, due to the more rapid undercutting and removal of softer rocks. 
In some cases, where this mode of undercutting occurs in hard 
flat ledges, just above high tide, the effects are very curious. I have 
observed such cases on the north side of Trunk Island and in other 
places. In some instances the portion of the ledge that is undercut 
may be only a few inches to a foot thick ; the outer border may run 
out to an irregular edge only an inch or two thick, while the under- 
cut groove may be several feet deep. This is quite unlike anything 
that happens on the open coast, where such projections would be 
soon broken off by the waves, even if they could be formed. 
ft. Erosion of the Outer Reefs and “ Flats” off the northern and 
western shores. 
The great barrier forming the broad outer border of the elliptical 
area, extending from off St. George’s all around to the Long Bar, 
the most southwestern of the reefs, is formed of a series of almost 
continuous broad patches of flat reefs. They are nearly flat on top, 
and are mostly submerged from one to ten feet below low-water 
mark. In certain places they are close to the surface or partially 
laid bare at low tide, and the seas break heavily over them in stormy 
weather. Such portions of the reefs are designated as “ breakers ” 
on the charts. In certain places patches of reefs, large or small, 
stand somewhat apart and outside of the main border line. Some of 
these, where the seas break heavily, are called “boilers,” as in the 
case of some of those to the east of Mills Breakers. A large number 
of outlying reefs exist outside the western border reefs.* 
Among the most dangerous of these detached reefs are the Chub 
Heads, 9 miles from the shore at Wreck Hill; Long Bar, of which 
the south part is 6 miles, W.S.W. from Gibb’s Hill Light ; and 
Southwest Breaker, on which the sea always breaks, and which les 
* Mr. A. Agassiz has given very full descriptions of many of these outer reefs 
and “‘ flats” from personal examination, and reference should be made to his 
memoir for more details. 
