108 
DEPTH AT WHICH 
Ch. IV. 
oscillations of level in the earth’s crust, or to the 
more precise but less important one of a cycle of 
years . 1 
Section III. 
On the Depths at which Beef-building Corals live. 
I have already described in detail the nature of the 
bottom of the sea immediately surrounding Keeling 
atoll ; and I will here describe with almost equal care, 
the soundings off the fringing-reefs of Mauritius. I 
sounded with the wide bell-shaped lead which Captain 
FitzRoy used at Keeling Island. My examination 
of the bottom was confined to a few miles of coast 
(between Port Louis and Tomb Bay) on the leeward 
side of the island. The edge of the reef is formed 
of great shapeless masses of branching Madrepores, 
■which chiefly consist of two species, — apparently 
M. eorymbosa and pocillifera , — mingled with a few 
other kinds of coral. These masses are separated from 
each other by the most irregular gullies and cavities, 
1 [See Dana, Corals and Coral Islands, ch. i. sec. iv. for additional 
facts relating to rate of growth of corals. Le Conte (Amer. Jonr. Sci- 
Ser. 3, vol. s. pp. 34 6) estimates that a Madrepora ( cervicomis ?) in 
shoal water at the Tortugas grew upwards at the rate of 3^ inches 
per annum. Duncan (Proc. Boy. Soc. xxvi. 133) estimates in the 
case of Lophohelia prolifera and Desmophyllum eristagalli growing in 
deep water to the north-west of Spain (522 to 550 fathoms) an in- 
crease upwards at the rate of 029 inches per annum. The result of 
later researches indicates considerable variation in the rate of 
growth, depending probably on species, locality, &c., and confirms the 
general conclusions of this paragraph.] 
