STRUCTURE OF CORAL REEFS. 113 



The rock of the submerged coral-heads is but a loose aggre- 

 gation of corals in the position of growth, except probably in 

 their lower portion, where the open spaces may be filled with 

 sand and fragments and all cemented together. 



The deposits of sand or coral mud over the bottom of the 

 seas outside of barrier reefs are sometimes of great extent. 

 These sands are the fine detritus which the return flow of the 

 breaker bears seaward ; and, in still deeper water, the deposits 

 should be mainly of the finest calcareous sand or mud — fit 

 material for impalpable compact limestones. The waters out- 

 side of the reef^ especially when moved by heavy tidal currents 

 or storms, are often milky with the coral sand ; and while the 

 coarser sand is dropped near the shores, the finer may be 

 carried for miles and distributed far out to sea. As Major 

 Hunt, in his observations on the Florida Reefs remarks, this 

 " white water" is one of the signs of proximity to a coral reef. 

 After storms, the white coral material subsides and the waters 

 become clear again. 



Mr. Jukes, who made special examinations of the Australian 

 reef region, and others in that vicinity, in H.M.S. F/y, states 

 that in the deeper waters outside of the great barrier, " and in 

 all the neighbouring East India seas, from Torres Straits, north 

 of Australia, to the Straits of Malacca, wherever the bottom 

 was brought up by the lead, it proved to be a very fine-grained, 

 impalpable, pale olive-green mud, wholly soluble in dilute 

 hydrochloric acid, and therefore essentially carbonate of lime. 

 The substance, when dried, looked much like chalk, excepting 

 in its greener tinge. How far this calcareous matter may be 

 due to foraminifers, rather than corals, is not known." 



Since the tidal waves on any coast that is gradually shal- 

 lowing have a landward propelling power, the coral sands are 

 mostly gathered about the reef, and generally are not to any 

 great extent lost in the depths of the ocean. The great oceanic 

 currents, like that of the Gulf Stream, might bear away the 

 lighter material for long distances, if it swept with full strength 

 over the shore reefs ; but it is generally true that such currents 

 are little felt close in shore. Notwithstanding the proximity of 



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