290 
APPENDIX II. 
and subsidence for a satisfactory ex]:>lanation of coral-reef 
formation. All-important among these causes are the 
prevailing winds and currents, the latter charged with 
sediment which helps to build extensive plateaux from 
lower depths to levels at which corals can prosper. This 
explanation, tested as it lias been by penetrating into the 
thickness of the beds underlying the coral reefs, seems a 
more natural one, for many of the phenomena at least, 
than that of the subsidence of the foundation to which the 
great vertical thickness of barrier-reefs has been hitherto 
referred. It is, however, difficult to account for the great 
depth of some of the lagoons — forty fathoms — on any other 
theory than that of subsidence ’ (p. 121). 
The author also describes the distribution of material, 
living and dead, on the Tortugas, the action of the waves 
in pounding up dead coral, molluscs, and other organisms. 
Thus a great quantity of calcareous ooze is formed (aided 
by the material which passes through the digestive cavity of 
holothurians, echinoderms, &c.). This silt, by i.s accumu- 
lation, kills the corals, which accordingly can only flourish 
where well ‘ scoured.’ The water is often chalk colour for 
a considerable distance from the reefs ; it is sometimes, 
after a heavy wind, discoloured for six to ten miles from 
the outer reefs. This process accounts for the scarcity of 
fossils. He also expresses the opinion that in this region 
the corals do not flourish at depths over six or seven 
fathoms, being probably choked by the ooze. 1 
1 In Three Cruises of the Blake, vol. i. p. 74, Prof. A. Agassiz 
states that ‘ all the evidence accumulated by Dana, Darwin, Ehrenberg, 
Quoy, and Gaimard tends to show that the limit of reef-building 
corals is to be found at about twenty fathoms.’ 
Prof. Agassiz’s views in regard to Florida do not appear to have 
met with universal acceptance among American men of science ; for 
instance, Mr. W. H. Dali (Geology of Florida, Amer. Journ. Sci., 
3rd ser. xxxiv. p. 161, 1877) says that in the southern part of Florida 
he saw no coral-rock or coral-reef formation : ‘ The coral formation 
