76 PHYSIOGNOMY OP PLANTS. 



of Captain Wilkes.) The presence of fluorine in the solid 

 parts of polypifers reminds us of the fluorate of lime in the 

 bones of fishes, according to the experiments of Morechini 

 and Gay Lussac at Rome. Silex is only found mixed in very 

 small quantity with fluorate and phosphate of lime in coral 

 stocks ; but a coral-animal allied to the Horn-coral, Gray's 

 Hyalonerna, has an axis of pure fibres of silex resembling 

 a queue or braided tress of hair. Professor Forchhammer, 

 who has been lately engaged in a thorough analysis of the 

 sea-water from the most different parts of the globe, finds 

 the quantity of lime in the Caribbean Sea remarkably small, 

 being only 247 parts in* ten thousand, while in the Categat 

 it amounts to 371 parts in ten thousand. He is disposed 

 to attribute this difference to the many coral-banks among 

 the West Indian Islands, which appropriate the lime, and 

 lower the per centage remaining in the sea- water. (Report 

 of the 16th Meeting of the British Association for the 

 Advancement of Science, held in 1846, p. 91.) 



Charles Darwin has developed in a very ingenious manner 

 the probable genetic connection between fringing or shore- 

 reefs, island-encircling reefs, and lagoon-islands, i. e., narrow 

 ring-shaped reefs enclosing interior lagoons. According to 

 his views these three varieties of form are dependent on the 

 oscillating condition of the bottom of the sea, or on periodic 

 elevations and subsidences. The hypothesis which has 

 been several times put forward, according to which the 

 closed ring or annular form of the coral-reefs in Atolls or 

 Lagoon Islands marks the configuration of a submarine 

 volcano, the structure having been raised on the margin of the 



