108 DEPTH AT WHICH Ch. IV. 



(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. corymbosa and pocillifera, — mingled with a few 

 other kinds of coral. These masses are separated from 

 each other by the most irregular gullies and cavities, 

 into which the lead sinks many feet. Outside this 

 irregular border of Madrepores, the water deepens 

 gradually to 20 fathoms, which depth generally is 

 found at the distance of from half to three-quarters of 

 a mile from the reef. A little further out the depth 

 is 30 fathoms, and thence the bank slopes rapidly 

 into the depths of the ocean. This inclination is very 

 gentle compared with that outside Keeling and other 

 atolls, but compared with most coasts it is steep. The 

 water was so clear outside the reef, that I could 

 distinguish every object forming the rugged bottom. 

 In this part, and to a depth of 8 fathoms, I 

 sounded repeatedly, and at each cast pounded the 

 bottom with the broad lead ; nevertheless the arming 

 invariably came up perfectly clean, but deeply in- 

 dented. From 8 to 15 fathoms a little calca- 

 reous sand was occasionally brought up, but more 

 frequently the arming was simply indented. In all 

 this space the two Madrepores above mentioned, and 

 two species of Astraea with rather large stars, seemed 

 the commonest kinds ; and it must be noticed that 

 twice at the depth of 15 fathoms, the arming was 

 marked with a clean impression of an Astraea. 



