102 PROCEEDINGS OF THE AMERICAN ACADEMY. 



have always had a great variety in dynamic habit and in the character 

 of their ejectamenta. Yet, in every one of them, the essential problem 

 is the same ; it refers to the mechanism by which heat is kept supplied 

 to the narrow, thread-like vents for long periods. To that problem, the 

 questions as to how sea-water or vadose water is absorbed by under- 

 ground magma, as to the dominance or subdominance of steam-explo- 

 sion at individual vents, and as to the physical differences in the 

 emanating lavas, are subsidiary. The problem of the Hawaiian vents 

 is, from this point of view, the problem of all vulcanism reduced to its 

 lowest terms. Here the gas-fluxing h}'j)othesis seems satisfactory. In 

 most other volcanic regions, where thick sediments are cut by the feed- 

 ing magma, or where heavy snows or rains wet the cones and, through 

 seepage, cause steam-explosions, the control by juvenile gas may be 

 obscured to the eye of the observer, but it still remains, in every case, 

 the true cause of continued activity. Kilauea and Mokuaweoweo, like 

 Matavanu and the vents in Reunion, teach us that steam-explosion is 

 an adventitious feature of vulcanism. Except abyssal injection itself, 

 the only indispensable process in central vents is quiet exhalation. 

 jXeither explosive drilling of the vent, nor ejection of lava, nor the 

 contacting of meteoric or marine water with hot lava is indispensable. 

 Each of these three processes is an expected effect of the slow emana- 

 tion of juvenile gas from main abyssal injections or from their satel- 

 litic offshoots. 



Magmatic Differentiation at Central Vents. 



The chemical variation exhibited in the lavas or pyroclastic materials 

 successively ejected at the normal vent offers a problem of special im- 

 portance. Volume for volume, this variability is much more striking 

 than it is in the average large intrusive body — stock, batholith, lac- 

 colith, or sheet. At present many petrologists favor the pure-differen- 

 tiation theory, which regards the splitting magma as primary, and 

 finds no place for notable assimilation of wall-rocks by the primary 

 magma. The writer believes that this question can only be cleared up 

 by an attentive study of the world's plutonic masses, and that, in the 

 nature of the case, its answer is not to be found at central vents. Field 

 and chemical relations point indubitably to the fact that wholesale as 

 similation has occurred in the subjacent bodies classed as stocks and 

 batholiths. Because of assimilation these masses generally have not 

 the basaltic composition of the primary abyssal injection. The visible 

 granite, diorite, or syenite represents the frozen top of an abyssal in- 

 jection which is there a more or less differentiated syntectic. The 

 lower part of each injection, approaching the substratum level, is prob- 



