166 P. A. Daly — Problems of the Pacific Islands. 



these Hawaiian vents, which are surely good strategic points 

 to be kept occupied in a campaign against our ignorance about 

 the interior of this planet. 



And, then, the final extinction of a volcano is a problem. 

 If the vent is kept open, directly or indirectly, by the emana- 

 tion of the earth's primitive gases, why is the end of a vol- 

 cano's life reached long before the earth's interior has lost 

 more than an infinitesimal fraction of its gaseous matter ? 

 Why has Kilauea's activity greatly decreased since 1820, when 

 the missionaries issued the first reports about it ? Why is the 

 adjacent Mauna Kea, or the well-known vent at Diamond 

 Head near the heart of Honolulu, without signs of life? Why 

 has Matavanu run its spectacular course in a half-dozen years ? 

 Is that vent already extinct or is it dormant? Why did hun- 

 dreds of volcanic giants on the Pacific floor become long since 

 extinct? The mere phrasing of these queries suggests the 

 ultimate way to answer them ; it is by comparative study at 

 the actual vents. 



Ultimately the explanation of the volcanic islands must be 

 rooted in a stable theory as to what underlies the ocean floor. 

 The initiation, rhythmic continuance, and final extinction of 

 vents, like the marked alignment of vents on the Pacific 

 charts, are related phenomena apparently applying the pre- 

 liminary injection of liquid lava into fissures within the solid 

 crust of the earth. Elsewhere I have advocated the view that 

 this primitive liquid is everywhere of basaltic composition. In 

 a subcrustal, basaltic stratum, which is at least locally eruptible, 

 I find most of the matter and energy which have built up the 

 Pacific volcanoes. On this view the volcanoes are the surface 

 expressions of great, molten, basaltic wedges locally thrust up 

 into the solid crust. The pulsing life of a volcano is dependent 

 on the size, gas content, and thermal energy of its feeding 

 wedge. Since each wedge is of limited length, width, and 

 height, ultimate extinction of every volcano is expected. 

 Whatever be the fate of this speculation, the detailed analysis 

 of the Pacific islands is destined largely to control the future 

 stable theory of volcanic mechanism. 



Origin of Limestone. 



A large proportion of the Pacific islands display no other 

 rocks than limestone. In now passing to a consideration of 

 the third principal kind of islands, I shall first touch on the 

 problem of limestone in general; and then, more in detail, 

 present leading questions about the reefs that have been built 

 by coral polyps and their organic associates. 



During the last few years some geologists have come to 

 doubt the prevailing explanation of limestone formations as 



