74 THREE CRUISES OF THE -" BLAKE." 



colored layer of decomposed vegetable and animal matter. The 

 Marquesas are covered by a thick growth of mangroves. 



Judging from my examination of the Tortugas reefs, it would 

 seem that corals do not thrive below a depth of from six to seven 

 fathoms. It is, of course, impossible to determine whether that 

 is then* bathymetrical limit, or whether they are killed by the 

 accumulation of ooze in the channels and adjacent slopes. We 

 find them confined, however, to the same shallow depths, along 

 the whole of the main reef to the northward ( Agassiz). Captain 

 Moresby also has shown that at a depth of ten fathoms in the 

 Maldive and Chagos archipelagoes the masses of living coral are 

 scattered at greater distances with intervening patches of smooth 

 white sand, and that at a slightly lower depth even these patches 

 merge into a smooth steep slope wholly bare of coral. All the 

 evidence accumulated by Dana, Darwin, Ehrenberg, Quoy, and 

 Gaimard tends to show that the limit of reef-building corals is 

 found at about twenty fathoms. On the Yucatan, as on the Flo- 

 rida Bank, the conditions favorable for coral-reef growth have 

 been produced, not by the uplifting of the continent, but by the 

 gradual rising of the bank itself in consequence of the accu- 

 mulation of animal debris upon it. The requisite level once 

 attained, reef -building corals (Fig. 45) would first establish 

 themselves on such spots as were most favorably 

 situated with reference to currents and prevailing 

 winds, both of which are essential to their healthy 

 growth, and thus the reef would be begun. 



How far the growth of corals is affected by 

 such local conditions is perhaps nowhere better 

 seen than in the smaller West Indian Islands. 

 Fig. 45. —Pelagic On the castcm side exposed to the prevailing 

 magnified. ' trade winds and washed by the great equatorial 

 currents, the corals flourish, while on the lee side they do not 

 exist at all. The whole eastern coast of Honduras, of Yucatan, 

 and of Venezuela, exposed to the same action and washed beside 

 by the Gulf Stream, is studded with coral reefs. To the action 

 of the Gulf Stream on the south coast of Jamaica and of Cuba 

 we must ascribe the presence of extensive coral reefs, and to the 

 same cause is undoubtedly due the great San Pedro Bank. The 



