A. E. Verrill—The Bermuda Islands; Geology. 95 
Sound, which is much shallower, and also of much later origin than 
the sound. But if the deeper sounds were excavated to their lower 
levels by running water, as it seems necessary to believe, then the 
channels must have been equally deep. That they should have been 
partly filled up after the subsidence is natural. Great masses of 
rock, broken by the storm waves from the high, bordering, sub- 
merged cliffs, would certainly fall into them from time to time. 
Shell sand and mud would collect in the interstices, and corals would 
grow on the fallen rocks, thus inevitably, but gradually, obstructing 
and filling them up. 
The action of the sea waves, in 10 to 20 fathoms of water, is 
almost inappreciative, and not sufficient to remove coarse sand or 
small stones. The tides, also, are here not strong enough to produce 
much effect at the bottom, even of shallower channels, though in 
some cases it is able to remove fine loose mud and keep the project- 
ing ledges bare. Reef corals, millepores, corallines, Gorgoniz, and 
other organisms grow well in these channels, where there are rocks, 
and their debris helps to fill up the channels. It is, therefore, not 
unlikely that some of these channels were once 30 to 50 feet deeper 
than at preseut. 
17. Broken Grounds outside the Reefs. 
The outer borders of the outer reefs usually fall off suddenly, 
like submerged cliffs, into water from 30 to 40 feet or more in depth. 
In many places the outer reefs are undercut, so that their tops over- 
hang their bases more or less, by the more rapid wearing away of 
the rocks below, owing either to their less hardened condition, or to 
less protection by living corals, alge, etc., or perhaps to both causes 
combined, in most localities. In other parts they fall away with 
steep slopes, or with a succession of steps, indicating layers of differ- 
ent hardness, like the cliffs along the shores. These submerged 
steep cliffs are rarely less than 30 to 40 feet high, and are more or 
less covered with a profusion of coral-heads, especially the brain- 
corals and astreans, Oculina, Millepora, gorgonians, corallines and 
nullipores, Sargassum and other large sea weeds, all of which have 
a great effect in retarding the erosion by the waves. The reef- 
corals, which are usually abundant and large to the depth of about 
25 to 30 feet, become small and scattered at about 40 feet, but the 
branching Oculinas and large Gorgonias extend down to 125 feet 
or more, where there are rocks for attachment. 
