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



the geology or the petrography adequately reported. Like the 

 andesites, the alkaline rocks are found in all the other primary 

 divisions of the globe, so that the petrologist busied in the 

 Pacific is really, here again, engaged on a question of world- 

 wide interest. 



Nature of volcanic action. — A few words concerning the 

 mechanism of volcanic action. Its working has been respon- 

 sible for the formation of nearly all the oceanic islands and 

 there more likely than anywhere else is our understanding of 



Fig. 1. 





\\ 







\ \hawaii 





\ \ MARSHALL 







* -—^CAROLINE \ 



X XFANNINC 





\ GILBERT 





£ Q U ATO R 



*"■ ^BISMARCK \ \ 







\ 



\50LOMON 



- ^N \ ELLICE 



\TOKELAU 



\MARQUESAS 



N. LOUI5IADE 



\SAMOA 



SOCIETY \ X 





\ NEW HEBRIDES 



0\ PAUMOTU 



\\ 



NEW CALEDONIAN. \LOYALTY 



^HERVEY 

 •s. 



N AUSTRAL 



Fig. 1. Trends in the Pacific islands (after J. D. Dana, Manual of Geology, 

 New York, 4th ed., 1895, p. 37). Scale 1:100,000,000. 



volcanic mechanism to be broadened and deepened. Figure 1 

 is a copy of J. D. Dana's well-known diagram showing the 

 trends of the volcanic chains on the Pacific floor. An idea of 

 the majestic proportions of each chain of cones is obtained if 

 one remembers that the Hawaiian line of high volcanoes is 300 

 miles in length. Do these alignments of the volcanic vents 

 directly indicate the step preliminary to all volcanism, namely, 

 the injection of subcrustal lava or magma into vertical fissures 

 opened through the crust? In other words, are the visible 

 vents so many holes long kept open in vertical abyssal dikes or 

 wedges of liquid lava ? If so, how is the heat maintained at 

 these openings for thousands of years, in spite of the proved, 

 enormously rapid radiation of heat at the craters? 



