Sect. T. THE GROWTH OF CORAL-REEFS. 
85 
our being able to assign any adequate cause for such a 
change . 1 
It has been a question with some naturalists, which 
part of a reef is most favourable to the growth of coral. 
The great mounds of living Porites and of Millepora 
round Keeling atoll occur exclusively on the extreme 
verge of the reef, which is washed by a constant suc- 
cession of breakers ; and living coral nowhere else 
forms solid masses. At the Marshall islands the larger 
kinds of corals (chiefly a species of Astraea, a genus 
closely allied to Porites), ‘ which form rocks measuring 
several fathoms in thickness,’ prefer, according to 
Chamisso , 2 the most violent surf. I have stated 
that the outer margin of the Maldiva atolls consists of 
living corals, (some of which, if not all, are of the same 
species with those at Keeling atoll), and here the surf 
is so tremendous, that even large ships have been 
thrown, by a single heave of the sea, high and dry on 
the reef, all on board thus escaping with their lives. 
Ehrenberg 3 remarks, that in the Pied Sea the 
1 I have left the foregoing paragraphs nearly as they stood in the 
first edition ; but, as stated in the Preface to the present work, Dana 
has shown that I have undervalued the importance of the mean 
temperature of the sea during the coldest season of the year, on the 
distribution of coral-reefs, as well as perhaps the injurious effects of 
recent volcanic action. But I cannot see that the absence of coral- 
reefs round certain islands in the Atlantic, for instance Ascension, 
St. Paul’s Rock, and Fernando Noronha, or from the shores of the 
Gulf of Panama, is explicable through any known cause. 
2 Kotzebue’s First Voyage (Eng. Transl.), vol. iii. pp. 142, 143, 
331. 
3 Ehrenberg, Ueber die Natur und Bildung der Corallen Biinke 
im rothen Meere, p. 49. 
