122 PROCEEDINGS OF THE AMERICAN ACADEMY. 



magmatic differentiation tends to favor explosiveness to an important 

 degree. Magmatic and phreatic explosions must be distinguished if 

 the tangle of vulcanological facts is to be unravelled. Though the 

 rise of hot magma into rocks charged with vadose or connate water 

 does often cause explosion, the steam-pressure produced by such 

 volatilized water can no more be regarded as the cause of vulcanism 

 than is the boiling of a kettle the cause of the heat in the stove. 

 The formation of the magma column, extending through the earth's 

 " granitic " and sedimentary shells to the surface, is the crucial prob- 

 lem. It is obviously a mere matter of detail whether or not the coun- 

 try-rocks at the upper end of the magmatic column are wet and 

 therefore explosive. 



The facts of volcanic geology seem, therefore, to co-operate with the 

 facts of plutonic geology in showing that the essential process in igne- 

 ous action on this planet is the rise of basaltic magma from the uni- 

 versal substratum along abyssal fissures in the earth's acid shell. The 

 whole group of conceptions emphasized in this paper may be summar- 

 ized in the name " substratum-Injection hypothesis." 



Massachusetts Institute of Technology, 

 Boston, Massachusetts. 



