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 their 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 débris 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 ‘eastern side exposed to the prevailing 
Porites Embryo, ; Д 
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 
