96 A. E. Verrili— The Bermuda Islands; Geology. 
The bottom slopes gradually form the bases of the submerged 
cliffs to the depth of 100 to 125 feet; beyond that it falls off more 
rapidly to about 300 feet, and then descends with a steep slope to 
6,000 feet or more. 
That portion of the bottom that les between 30 or 40 feet and 
about 120 feet is irregular, rough, and more or less completely cov- 
ered with ledges and blocks of stone, interspersed with patches of 
white shell-sand. As the depth increases the shell-sand predominates 
more and more, until at about 100 to 125 feet, and beyond, it covers 
most of the bottom. 
The rough rocky zone, between about 40 and 120 feet, is called 
the “broken ground” by the fishermen. Many of the rocks seat- 
tered over this slope are doubtless great and small masses that have 
been torn from the outer edges of the reefs by the violent sea-waves, 
during the thousands of years since they were submerged. Others 
are probably eroded ledges of xolian limestone. These rocks are 
more or less covered, especially in the shallower parts, with an 
abundance of living organisms, such as corallines, nullipores, and 
other alge ; large branching corals of the genus Oculina ; and large 
gorgonians, such as the sea-fan, Gorgonia flabellum ; the sea-plume, 
G. acerosa ; Plexaura flexuosa; Plexaurella crassa, Verrucella, ete. 
(See also chapter 29.) 
This zone of ‘broken ground” is often two to three and a half 
miles wide off the eastern, northern, and western reefs ; but usually 
only one-half to one mile wide off the reefs of the southern side. 
It undoubtedly represents what were once the low lands, shores, 
and shallows of Greater Bermuda, to which have been added 
immense quantities of debris derived from the erosion and tearing 
down of the outer reefs by the violent oceanic waves that beat on 
the outer edges of these upright reefs with immense force during 
storms. 
The character of the bottom over this zone, its slope, and its depth 
indicate a submergence of at least 100 feet, as do the cuts and 
sounds described above. 
18. Argus and Challenger Banks. 
The present very uneven surfaces of Argus and Challenger Banks 
are like those of Bermuda, and indicate erosion when they stood 
above sea-level. Some considerable parts of their summits are now 
180 to 240 feet beneath the sea ; other parts (Argus Bank) are only 
8 feet (see fig. 7). Erosion by waves on such sunken banks would 
