A. FE. Verrilli— The Bermuda Islands; Geology. 53 
abundant growth of corals at Bermuda, and a less profound erosion 
of the submerged limestone banks and cliffs on which the recent 
corals have grown. In view of this, I have previously suggested 
that such a structure as that of the Bermuda reefs should be called 
a pseudatoll. 
Probably the position of the more elevated rim of limestone reefs, 
and the ancient sand-dunes of which they are remnants, was largely 
determined by still older coral reefs of Tertiary age, but this cannot 
be ascertained at present. 
4. Voleanic Character of the Bermuda Foundation. 
All geologists admit that the Bermudas rest on the flattened and 
eroded summit of a vast submarine volcano. The geological period 
when this volcano was last active is, of course, very uncertain. It 
is, however, most reasonable to suppose that it corresponded in time 
with the last great volcanic eruptions of the nearest American main- 
lands. This would imply that the Bermuda voleano was formed or 
completed during the Triassic period or at its close. During that 
period, and at its close, immense outbursts of volcanic rocks took 
place all along the eastern coast of North America, ‘from North 
Carolina to Nova Scotia, giving rise to enormous trap-dykes, such as 
the Palisades of the Hudson; Mount Tom, Mt. Holyoke, Meriden 
Hills, and numerous other extensive outflows along the Connecticut 
River valley; and also the vast series of dykes in Nova Scotia, espe- 
cially along the east side of the Bay of Fundy. As the Nova Scotian 
regions of eruption are only about 675 miles north of Bermuda and 
the immense dykes have a nearly north and south direction, it is not 
unlikely that the outburst at Bermuda was in direct relation with 
those of Nova Scotia. 
The great Bermuda volcano has a height of about 15,000 feet, for 
the surrounding ocean is about 2500 fathoms deep. Its slope on all 
sides is very steep. Its form and height prove that it is a volcano. 
This is confirmed by the remarkable magnetic variations detected 
by the officers of the ‘“ Challenger” in different parts of the islands, 
which could hardly be caused by anything except iron-bearing vol- 
canic rocks not far beneath the surface. 
“The observations made by the Expedition showed that the varia- 
tion differed in various parts of the island as much as 6°, ranging 
from 4° W. to 10° W., the smallest amount being found at a small 
islet just under the lighthouse on Gibb’s Hill, and the greatest at 
