138 



FORMATION. 



band of grit-based coralline, which runs meridion- 

 ally, and is most remarkable on the eastern side. 

 This portion, featureless and thinly inhabited, is 

 protected from the dangerous swell and the fury 

 of the Indian Ocean by a broad reef and scat- 

 tered rocks of polypidoms. The band thins out 

 to the north and south : in the centre, where it 

 is widest, the breadth may be three to four miles, 

 and the greatest height 400 feet. The coral-rag 

 is mostly white and of many shapes, like fans, 

 plants, and trees : the most usual form is the 

 mushroom, with a broad domed head rising from 

 a narrow stem. The texture is exceedingly re- 

 ticulated and elastic; solid masses, however, 

 occur where neighbouring rocks meet and bind — 

 hence the labyrinth of caverns, raised by secular 

 upgrowth and preserving the original formation. 

 The ground echoes, as in volcanic countries, hol- 

 low and vault-like to the tread ; the tunnels are 

 frequently without issue for drainage, and when 

 the rain drips in, the usual calcareous pheno- 

 mena, stalactites and stalagmites, appear. Many 

 of these caves are found on the coast as well as 

 on the island. The carbonate of lime is very pure, 

 and contains brown or yellow-white crystals. 



A stony valley, sunk below the level of both 

 flanks, is said to bisect the island from north 



