i6 



INDEX TO THE PACIFIC ISLANDS. 



canic, eruptions frequently occurring in some part of that territory. A region of such 

 marked volcanic character might be expected to exhibit the concomitant phenomena of 

 earth movements, both earthquakes and the grander if less obtrusive movements of 

 elevation and depression, and it was the latter change in level that gave Darwin the 

 foundation of his ingenious theory of the formation of 



Coral Islands. — Most important, both from a geological and a zoological 

 standpoint are the buildings of the coral-forming polyp. Throughout the portion of the 

 Pacific between the dotted lines on the diagram of this ocean ( Fig. 2) this minute animal 

 has by the force of numbers greatly increased the area of habitable land, made harbors 



FIG. 2. DIAGRAM OF ISOCHRYMES BETWEEN WHICH REEF-CORALS OCCUR. 



possible, and changed if not created currents in the equatorial sea. This is not a work 

 now complete or of paroxysmal or intermitent nature, but it is a work of the present 

 dav, like the aeolic erosion of valleys and shows no sign of diminution. 



While the other great agency in the formation of the intra-Pacific lands, vulean- 

 ism, seems to be diminishing from Hawaii to the Solomon Islands, the coral polyp, all 

 unconscious, it may be, as the volcano of its mighty work, goes on building up reefs 

 which in time become habitable islands. 



As a certain degree of warmth is needed for the life as well as growth of reef- 

 forming corals, and not all corals come into this class, the boundaries of the coral 

 region both north and south of the equator will be determined by the isocryme (or line 

 of equal cold ) of 68 F., colder water preventing their growth, and their activity in- 

 creasing with the mean temperature. In the hotter water under the equator the teni- 



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