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 ; uamel}^, on steep banks, or 

 on the slojjes which are scoured by tides, or on flats at con- 

 siderable dejiths, 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 deposited 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 ujjon the exposed 

 slopes of the reefs destroy, even on calm days, large quantities 

 of corals which have been weakened by the borings of moUusks, 

 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- 

 lono-ed 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 

 which 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 Bernini, we have a huge muddy bot- 



