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144 ACTION OF THE SEA. 
aware Bay, it was proved by measurement that, 
from the year 1804 to 1820, the annual encroach- 
ment of the sea averaged nhie feet ; and at Sulli- 
van's Island, near Charleston, South Carolina, the 
sea carried away a quarter of a mile of land in three 
years ending in 1786. 
In relation to Boston Harbour, Professor Hitch- 
cock remarks : " Here are numerous picturesque 
islands, the inner ones, nearly as far as Boston 
Light, being composed chiefly of diluvium ; though 
on their shores, at a low level, not unfrequently we 
find argillaceous slate and other rocks that occur 
on the main land. But all the islands outward from 
the Great Brewster are merely naked masses of 
rock ; and it would be natural to infer that the di- 
luvium had been removed from these, even if we 
did not actually detect the process. But on the 
Great Brewster the work is going on before our 
eyes. Its eastern side is a high bank of diluvium, 
obviously wasting away by the action of the waves 
that roll in upon it from the wide Atlantic, while 
the extensive beach along its southern side is com- 
posed of the materials that have been swept away 
from its outer coast. The same process is seen 
going on upon the outer side of several other isl- 
ands ; and on Deer Island an extensive wall of 
stone has been erected by the United States gov- 
ernment to arrest the progress of this degradation ; i 
which, if continued much longer there, would lay 
open the inner part of Boston Harbour to the fury | 
of the northeasterly storms."* j 
Professor H. inclines to the belief that the whole I 
of Boston Harbour, now dotted with small islands, 
was formerly one piece of solid land; "for," he I 
remarks, " w^hen we see so many islands scattered 
over its bosom, which seem obviously the wrecks j 
of one contmuous diluvial formation, and perceive ' 
that the rocks, wherever they occur, are only a con- 
* Report of Geology of Massachusetts, p. 125. 
