GEOGRAPHICAL DISTRIBUTION. 317 



much as it is contracted on the western coasts. Moreover, the 

 trend of the coast and its capes produce other modifications in 

 the direction of the currents, the most of which are apparent 

 in the actual distribution of coral reefs. On the shores of 

 the continents it is observed that there are few extensive 

 reefs, and the coasts on which they occur are those which, 

 owing to the dryness of the climate, have no great rivers to 

 pour freshwater and detritus into the sea. Thus the influence 

 of continental waters and detritus on the distribution of reefs, 

 is shown to be very marked. But about the Pacific Islands, 

 where streams are small, the same cause has had little effect, 

 seldom doing more than modifying somewhat the shores and 

 bottom of a harbor. It has been further demonstrated that 

 in different groups, as the Ladrones, Sandwich Islands, Navi- 

 gators, New Hebrides, there is an inverse relation between 

 the extent of reefs and the evidences of recent volcanic ac- 

 tion in the island ; and that the largest reefs exist where 

 there is no proof of former igneous action, or where it has 

 long ceased. The existence of large reef-islands in open seas 

 Avhere the neighboring lands are mostly destitute of coral 

 reefs, harmonizes with the conclusions announced, since such 

 islands are in general removed from the deleterious influences 

 just mentioned ; yet it is very possible that in some cases of 

 this kind the region of the open sea may have undergone a 

 subsidence not experienced equally by the lands either side. 



The modifications of form and interruptions of reefs, aris- 

 ing from abrupt or sloping shores, and tidal or local cur- 

 rents, have also been exemplified. 



