OE0011APHICAL DISTRIBUTION. 301 



Pacific Ocean. Atlantic Ocean. 



East side of ocean — Northern, Lat. 21° N Lat. 10° N. 



Southern. 4° S. 5° S. 



West side of ocean — Northern. 15° N. 26° N. 



....Holland. ) M . & 



j 30° S., N. 

 I 29° S., Afi 



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

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



In the Pacific twenty-five degrees wide on the west coast 

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



In 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 off 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 seem- 

 ing 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. 114). The unfavorable character of sandy or muddy 

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

 waters have also been stated (p. 119), 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 influ- 

 ence are numerous throughout the Pacific. The existence of 

 narrow reefs, or their entire absence, may often be thus ac- 

 counted for. For example, in the Hawaian Group, the island 

 Hawaii, still active with volcanic fires, has but few traces of 

 corals about it, while the westernmost islands, which have 



