56 PROCEEDINGS OF THE AMERICAN ACADEMY. 



First, basaltic magma, rising from such depth nearly to the earth's 

 surface, must undergo an average expansion ranging between 1.5 and 6 

 percent.** A small part of this expansional energy may be directly 

 available for opening fissures in the shell of compression, with conse- 

 quent extrusion at the surface or development of laccolithic or other 

 bodies within that shell. 



Secondly, some superheat might be expected in each thick abyssal 

 injection, at levels where the pressure is much less than the 11,000 

 atmospheres of the original substratum level. The conditions for the 

 superheat have been briefly discussed in a previous paper. ^^ Super- 

 heat in large abyssal injections means power of assimilating the wall 

 rocks either marginally, or through magmatic stoping (forming syntec- 

 tic magmas) ; the batholithic type of intrusion results. Thin abyssal 

 injections, rapidly chilled, are relatively or absolutely incapable of such 

 solution of foreign material ; those injections form the other great class, 

 typified by ordinary dikes. Each of these types is capable of developing 

 offshoots of the laccolithic, chonolithic, or sheet order, so that satellitic 

 injections of primary basalt or of syntectic matter are formed. This 

 simple but necessary division into assimilating and non-assimilating 

 injections is of first-class importance in volcanic theory, and underlies 

 the reasoning of the present paper. 



Thirdly, magma which has been forced from the substratum level to 

 levels where the pressure is 10,000 atmospheres less, must have com- 

 pletely altered conditions of equilibrium for the juvenile gases. These 

 include hydrogen, sulphur gas, carbon monoxide, carbon dioxide, chlo- 

 rine, nitrogen, and other gases, elementary or in combination. The 

 theory of physical chemistry indicates that the dissolved volatile con- 

 stituents must, in such a case, slowly diffuse upward, in order to re- 

 establish equilibrium. There is thus a tendency to saturate and then 

 supersaturate the upper part of the magma with juvenile gases ; if by 

 the mere change of pressure the magma is supersaturated with one or 

 more of the gases, bubbles must form and these must slowly rise. If 



tasy," Coast and Geodetic Survey, Washington, 1910. If the top of the 

 liquid (non-crystallized) basaltic substratum were at a depth as great as 122 

 kilometers, this change in premises would make no essential difference in the 

 argument of the present paper. It may be noted that John Milne (Nature, 86, 

 124 (1911)), using Rizzo's results in studying the velocity of the earthquake 

 waves which were recently so disastrous at Messina, has estimated the thickness 

 of the earth's " crust " to be 44 kilometers. 



" See Amer. Jour. Science, 22, 201 (1906). 



^•^ Amer. Jour. Science, 26, 33 (1908). Cf. J. Joly, Radioactivity and Geol- 

 ogy, London, 1909, pp. 103-109, where there is an important discussion of 

 one phase of this subject. 



