GEOGRAPHICAL DISTRIBUTION. 257 



It follows from the above, that while the coral-reef seas are 

 about fifty-six degrees wide in mid-ocean, they are 



/// the Pacific twenty-five degrees wide on the west coast of 

 America, and forty-five degrees on the Asiatic side. 



/// the Atlantic^ about fifteen degrees wide on the African 

 coast, and forty-eight degrees on the coast of America. 



If we reckon to the extremity of the bend in the Gulf Stream, 

 the whole width of the coral-reef sea oif the east coast of 

 America will be over sixty-four degrees; while off the west 

 coast of America the width is hardly eighteen degrees. It is 

 obvious that these facts enable us to explain many seeming 

 anomalies in the distribution of coral reefs. 



The other causes which influence the distribution of reefs 

 operate under this more general one of oceanic temperature, 

 that is, within the coral-reef boundary lines. The effect of a 

 deep abrupt coast on the distribution of reefs has been pointed 

 out (p. 89). The unfavourable character of sandy or muddy 

 shores, and the action of detritus, marine currents, and fresh 

 waters have also been stated (p. 93), and it is not necessary 

 to touch again upon these points. 



Not less striking are the effects of volcanic action in pre- 

 venting the formation of reefs ; and instances of this influence 

 are numerous throughout the Pacific. The existence of narrow 

 reefs, or their entire absence, may often be thus accounted 

 for. For example, in the Hawaian Group, the island of Hawaii, 

 still active with volcanic fires, has but few traces of corals about 

 it, while the westernmost islands, which have been longest 

 free from such action, have reefs of considerable extent. The 

 island of Maui exemplifies well the same general fact. The 

 island consists of two peninsulas : one, the eastern, recent 

 volcanic in character, with a large crater at summit ; and the 

 other, the western, presenting every evidence, in its gorges and 

 peaks and absence of volcanic cones, of having become ex- 

 tinct ages since. In conformity with the view expressed, 

 the coral reefs are confined almost exclusively to the latter 

 peninsula. Other examples are afforded by the Samoan or 

 Navigator Islands. Savaii abounds in extinct craters and lava 



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