122 
THEORY OF THE FORMATION 
Ch. V. 
What cause, then, has given to atolls and barrier- 
reefs their characteristic forms ? Let us see whether 
an important deduction does not follow from the 
following facts, — first, that reef-building corals only 
flourish at a very limited depth, — and secondly, that 
throughout areas of vast dimensions, none of the 
coral-reefs and coral-islets rise to a greater height 
above the level of the sea than that attained by matter 
thrown up by the waves and winds. I do not make 
this latter statement vaguely ; I have carefully sought 
for descriptions of every island in the inter-tropical 
seas ; and my task has been in some degree facili- 
tated by a map of the Pacific, corrected in 1834 by 
MM. D’Urville and Lottin, in which the low islands 
are distinguished from the high ones (even from those 
much less than a hundred feet in height) by being 
written without a capital letter. 1 I have also ascer- 
tained, chiefly from the writings of Cook, Kotzebue, 
Bellingshausen, Duperrey, Beechey, and Lutke regard- 
ing the Pacific ; and from Moresby 2 with respect to the 
Indian Ocean, that in the following cases the term 
‘ low island ’ strictly means land of the height com- 
1 I have detected a few errors in this map, respecting the heights 
of some of the islands, which will be noticed in the Appendix, where 
I treat of coral-formations in geographical order. To the Appendix, 
also, I must refer for a more particular account of the data on which 
the following statements are grounded. 
2 See also Captain Owen’s and Lieut. Wood’s papers in the Geo- 
graphical Journal on the Maldiva and Laccadive Archipelagoes. 
These officers particularly refer to the lowness of the islets ; but I 
chiefly ground my assertion respecting these two groups, and the 
Chagos group, from information communicated to me by Captain 
Moresby. 
