Geology and Natural History. 313 
I.) The second sentence of the Preface is as follows: 
“In this work [Dana’s Corals and Coral Islands] he [the author] 
justly says that I have not laid sufficient weight on the mean tem- 
perature of the sea in determining the distribution of coral reefs ; 
but neither a low temperature nor the presence of mud ban 
accounts, as it appears to me, for the absence of coral reefs through- 
out certain areas; and we must look to some more recondite 
cause.” 
h 
distribution of coral reefs. In his discussions on the distribution 
a to the coldness of the currents from the south, but th 
e ; 
region referred to. Thus the cause is set aside even for the seas 
along the Peruvian coast, although the mean winter temperature 
causes limiting growth and the distribution of reef-making 
corals and coral reefs, which I have discussed and applied in my 
work, are seven in number. 
(1.) Marine temperature. ; 
(2.) Fresh and impure waters from the entrance of large rivers ; 
and muddy bottoms. hl 
(3.) Deposition of sediment borne by rapid tidal currents. 
s may grow—a commo 
condition along bold coasts, and often explaining, as I have found, 
the contrasts between the reef-bordered and open coasts of the 
Same island. : : 
(5.) Exposure to the heat of submarine voleanic eruptions (pp. 
299-317 
; i i i id for the 
(6.) The progressing coral-island subsidence too rapid for t 
polyps to ep the reef well at the surface, if at all (p. 270): which 
a * 
