84 THREE CRUISES OF THE “BLAKE.” 
and plant with a calcareous skeleton, of course prevents the 
growth of corals in positions which are not well scoured, either 
by the action of the tides or by that of the prevailing winds. 
The corals when alive are gradually buried under this mass 
of material constantly passing over them, and held in suspen- 
sion. They flourish, therefore, only where the disturbing ele- 
ments are reduced to a minimum; namely, on steep banks, or 
on the slopes which are scoured by tides, or on flats at con- 
siderable depths, over which a large body of water can freely 
pass, whether brought by the tides or driven by the winds. In 
such cases the corals can grow gradually towards the surface as 
fast as the sediment dog aliis has closed up the circulation of 
the lower levels. The quantity of calcareous matter held in 
suspension in the water in the vicinity of a reef, and on the reef 
itself, is very great. The breakers pounding upon the exposed 
slopes of the Шер destroy, even on calm days, large quantities 
of corals which have been weakened by the borings yr mollusks, 
annelids, echinoderms, and sponges. On windy or stormy days 
the powdered fragments are driven far and wide, turning the 
surrounding water to chalk color for a considerable distance 
from the reef. It is not an uncommon thing, after a blow, to 
come upon this water discolored by the fine calcareous silt, to a 
distance of six to ten miles from the outer reef. After a pro- 
longed storm I have seen between two and three inches of fine 
silt deposited in the interval between two tides. The limitation 
of coral-reef growth to shallow depths may be due to the fact 
that the ooze held in suspension rapidly sinks towards the bot- 
tom, the surface water remaining clear. The rapidity with 
viola the corals are choked readily explains why they must of 
necessity have a limited vertical distribution depending upon 
local causes. This is well shown along the sections of the Tor- 
tugas. Off the Marquesas, and along the line of the main reef, 
we find corals living and flourishing at a much greater depth, and 
there seems to be no simpler explanation of the limited bathy- 
metrical range than that of the baneful action of the silt near 
all reefs. That the silt is carried on the bottom by currents and 
waves is well known, and on the bottom of the Gulf Stream, to 
the north of the Straits of Bemini, we have a huge muddy bot- 
