56 A. FE. Verrill—The Bermuda Islands; Geology. 
some feeble effects to the depth of 100 feet or more, but hardly 
sufficient to move anything more than loose material like fine sand 
and mud. 
George’s Bank and Nantucket Shoals, off Cape Cod, maintain 
themselves in the face of the most violent storms. Although com- 
posed only of loose sand and gravel, their shallowest parts rise to 
within 25 to 30 feet of the sea-level. This indicates that the erod- 
ing action of the waves decreases very rapidly, even at such depths. 
The Argus and Challenger cones were evidently truncated and 
roughly levelled by the erosion of the waves, but at the present 
time they are depressed so far beneath the sea that coral reefs do 
not grow upon them. Possibly they may have been dry land, with 
sand dunes and corals like those of Bermuda, in the period of Greater 
Bermuda. If so, the subsequent subsidence and simultaneous ero- 
sion of the limestones could have reduced them to their present 
depths. 
If Jurassic or Tertiary coral reefs existed here, as is quite prob- 
able, they would certainly have grown best around the borders of 
the banks and shoals. Thus they might have initiated the atoll-like 
structure that has prevailed subsequently. 
It is possible that during some of the former geologic periods, after 
the cones were formed, there may have been long periods of subsi- 
dence, in which the depth of water over them became too great for 
the growth of coral reefs,* as is now the case at the Argus and 
Challenger Banks. 
5. Emergence of the Land. 
At some period, perhaps after the close of the Miocene, when we 
know that many of the West Indian islands, with their Miocene 
corals, were upraised, as well as the eastern coast of the United States; 
or perhaps still earlier, in the Eocene, the Bermuda reefs and shoals, 
whether of coral or not, were so much raised that they formed dry 
land.t No doubt this land at first formed a group of low islands 
* Deep artesian borings at Bermuda might determine these questions with 
certainty. No doubt this will eventually be undertaken, as has been done else- 
where. 
+ That the dry land was as old as the middle Tertiary is probable, because of 
the long time that must have been required for the evolution of the endemic 
genus Peecilozonites, with at least seven very diverse species that we find already 
there in the Pliocene. There must have been many earlier ancestral species 
that are unknown to us. See Paleontology. 
