72 FKINGING KEEFS. Ch. III. 



outside not yielding many fragments to the breakers. 

 On the windward side, however, of the Mauritius, two 

 or three small islets have been formed. 



It appears, as will be shown in the ensuing chapter, 

 that the action of the surf is favourable to the vigorous 

 growth of the stronger corals, and that sand or sedi- 

 ment, if agitated by the waves, is injurious to them. 

 Hence it is probable that a reef on a shelving shore, 

 like that of Mauritius, would at first grow up, not 

 attached to the actual beach, but at some little distance 

 from it ; and the corals on the outer margin would be 

 the most vigorous. A shallow channel would thus be 

 formed within the reef ; and this channel could be filled 

 up only very slowly with sediment, for the breakers 

 cannot act on the shores of the island, and they do 

 not often tear up and cast inside fragments from the 

 outer edge of the reef, whilst every streamlet carries 

 away its mud in a straight line through breaches in 

 the reef. But a beach of sand and of fragments of 

 the smaller kinds of coral seems, in the case of Mauri- 

 tius, to be slowly encroaching on the shallow channel. 

 On many shelving and sandy coasts, the breakers tend 

 to form a bar of sand a little way from the beach, with 

 a slight increase of depth within it : for instance, Captain 

 Grey 1 states that the west coast of Australia, in lat. 

 24°, is fronted by a sand bar about 200 yards in width, 

 on which there is only two feet of water ; but within 

 it the depth increases to two fathoms. Similar bars, 



1 Captain Grey's Journal of Two Expeditions, vol. i. p. 369. 



