128 A. E. Verrill—The Bermuda Islands; Geology. 
than it would otherwise be. Allowing the average to be even an 
inch in 25 years, it would have required at least 25,000 to 30,000 
years for the sea to have eroded the high cliffs of the islands facing 
on Castle Harbor to the extent that they have been eroded. 
I did not obtain any reliable data as to the rate of erosion of the 
exposed cliffs of the south shore, except the observation that on 
Castle Island the ancient sea-walls of the forts were often built with 
their foundations on the edges of the cliffs (fig. 1) or even on a 
shelf of limestone some distance below the original brow of the 
seaward cliffs, and they have not yet been undermined, but stand 
firmly where they were put many years ago.* So, likewise, the 
Cathedral Rocks show scarcely any changes since the earlier photo- 
graphs were made, about 30 years ago. I compared an excellent 
photograph, made by Mr. J. B. Heyl, about 1875, with the condition 
of the rocks as they were in 1901, and could find no changes worthy 
of note. 
Figure 31.—North Rocks. From a photograph taken by Mr. J. B. Heyl in 1875. 
North Rocks in former times. 
A comparison of several photographs taken at various times within 
the past thirty years shows but little modern alteration in the North 
Rocks, but some very severe storm might suddenly overthrow them. 
They are situated near the extreme edge of the outer reefs, about 
* Although the first stone fort was built here about 1620, the present ruined 
walls may date no farther back than 1812. 
