CORALLINES. 179 



It follows from this that madreporic isles cannot exist in all seas ; 

 that they only have their birth in the Torrid zone, or at least near 

 the Tropics, for it is only in these regions where the warmth exists, 

 so necessary to their development, that the madrepores show them- 

 selves in greatest abundance. 



The great field of madreporic formations, in short, is found in 

 the warm parts of the Pacific Ocean. It is from this point, as from 

 a common centre, round which are ranged the series of madreporic 

 isles and islets, that it will be useful, in concluding this chapter, to 

 trace their geographical distribution. We borrow the materials for 

 this from Milne Edwards's tableaux of their distribution in the 

 principal seas of the world. 



It is, as we have said, only in the warm parts of the Pacific Ocean 

 that the great mass of these islands are found. They give birth 

 towards the south to the group of atolls known as the archipelago 

 of the Bashee Islands, the extreme limit of the region being the Isle of 

 Ducie. A multitude of other islands of the same nature are sparsely 

 scattered over the sea, up to the east coast of Australia. There 

 are enormous areas here, in which every single island is of coral for- 

 mation, and is raised to the height at which the waves can throw up 

 fragments. The Kadack group is an angular square, four hundred 

 miles long by two hundred and forty broad. Between this group and 

 the Low Archipelago itself, eight hundred and forty miles by four 

 hundred and twenty, there are groups and single islands covering a 

 linear space of more than four thousand miles. To the north of the 

 Equator, the archipelago of the Caroline Islands constitutes a very 

 considerable group of madreporic formation, comprehending upwards 

 of a thousand, extending in a broad belt over nearly forty degrees of 

 longitude. On the other hand, all along the coast of the American con- 

 tinent, round the Galapagos and the Isle of Paques, we find no trace of 

 them. The reason assigned is, that in these regions a great current of 

 cold water, flowing from the Antarctic Pole, so much lowers the tempera- 

 ture of the sea, that the zoophytes no longer possess the requisite vigour. 



We still meet with atolls in the Chinese Seas, and madreporic 

 barrier reefs are abundant round the Marianne and Philippine Islands. 

 These marginal reefs form also an immense tract, from the Isle of 

 Timor, along the south coast of Sumatra, up to the island of Nicobar, 

 in the Bay of Bengal. 



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