A. FE. Verrill—The Bermuda Islands; Geology. 57 
along the rim of the partly enclosed lagoon, as is almost always the 
case with small oceanic islands of this type. Their sandy beaches 
and flats, alternately covered by the tide and exposed to the sun, 
afforded an abundance of dry shell-sand. From this time onward 
the shell-sand, derived mainly from the life and death of myriads of 
small mollusks on and about the reefs and shoals, must have been 
drifted by the winds, so as to form hillocks and sand-dunes, grad- 
ually increasing the height and extent of the islets and eventually 
uniting them together into larger ones. It is probable that this was 
favored and accelerated by the continued and gradual uprising of 
the volcanic basis, during a long period of time. But it is possible 
to account for much of the subsequent great growth of the islands, 
even without much elevation of the sea bottom, beyond what was 
necessary to lay bare the extensive shoals of fine shell-sand, periodi- 
cally covered by the tide.* 
6. Evolution of Greater Bermuda; Pliocene Bermuda. 
From the evidences derived from the subsequent subsidence, it is 
probable that the highest sand dunes, eventually, in one period at 
least, attamed the height of over 450 feet. It is hardly probable 
that this was due wholly to the drifting of the sand to that height, 
though it is not impossible. It seems more probable that the emer- 
gence of the land continued while the great sand dunes were form- 
ing. In that case the higher and larger sand dunes would also be 
the older ones and the deposits at the center and summit would be 
the oldest. If the height were wholly due to sand-drift, then the 
upper layers at the summits would be of later origin. The character 
of the rocks indicates, but does not prove, that the upper and central 
parts of the higher hills are the oldest. But fossils have not yet 
been found in them. Thus along continued period of emergence 
-was probable, with a constant loss of materials from the tops of the 
hills. 
No doubt a very large amount of material has been removed from 
all the hills through solution and by mechanical erosion by rains, so 
that 450 feet for their greatest former elevation is probably too low 
an estimate. 
* Subsequent subsidence has buried the first formed limestones deeply beneath 
the sea,—probably at least 100 feet. We know the nature of the submerged 
rock to about 50 feet deep at Ireland Island dock, where it is u sand-drift lime- 
stone, associated with red clay soil. 
