A. FE. Verrill—The Bermuda Islunds,; Geology. 127 
St. George’s Harbor and Castle Harbor are examples of the same 
kind of erosion done on a larger scale and much longer ago; probably, 
also, much aided by subsidence of the land. The two causes operate 
together and in most of the older cases cannot be considered sepa- 
rately. 
21. Rates of Erosion by the sea ; modern changes:slow. 
Most observers, seeing the evidence of great erosion on all sides, 
and considering the softness of the rocks, have naturally supposed 
that the erosion has taken place far more rapidly than is the case. 
In my studies of the rate of erosion by the waves, as shown on 
the masonry of the causeway leading to St. George, and in other 
places, this rate of erosion was found to be unexpectedly slow, 
under ordinary conditions, owing to the absence of ice and frost; 
also because there are no deposits of very hard sand, gravel, and 
pebbles on the shores, which the storm waves can pick up and use as 
tools of destruction, by dashing them against the bases of the cliffs 
and against each other, as they do on our rocky coasts. It is only 
during the severe storms and hurricanes, which occasionally occur, 
that rapid erosion is accomplished. 
The causeway between the main island and St. George’s was com- 
pleted in 1871.* It was about a mile long, and fairly well built of 
native limestone blocks of considerable size. It included an iron 
drawbridge and several smaller bridges, under which were strong 
tidal currents, flowing in and out of St. George’s Harbor and Castle 
Harbor. It is so situated in the passage between the islands that it 
is partly sheltered by the outlying small islands and reefs, and ordi- 
narily it is not exposed to the full violence of storms. By an exami- 
nation of the masonry of this causeway, in 1898, at various places, I 
found that during the 27 years that it had been built, the erosion by 
the sea rarely amounted to an inch in depth, where most active, and 
the average erosion was less than half an inch, between tides ; most 
of this, also, had evidently been effected within the first few years 
after its erection, befqre the stones had acquired their hard superficial 
coat of infiltrated calcite. It is true that these stones were selected 
from the harder beds of limestone and therefore had more than the 
average resisting power, but after any of the soft limestones become 
infiltrated by calcite, the surface is resistant, so that the differences 
in power of resisting erosion by the sea, between tides, is much less 
* It was totally destroyed by the great hurricane of Sept., 1900, but has since 
been rebuilt in a different way. 
