522 N. L. BOW EN 



lo per cent of the mass of magma would be sufficient to wipe out this 

 superheat, for the original temperature of blocks even at this quite 

 considerable depth would be only 200°. 



This, too, is quite apart from the amount of heat that must be 

 lost by the magma in heating up a large amount of wall rock other 

 than the immersed fragments. If we consider greater depths, where 

 the surroundings would be at a higher temperature, it must be 

 remembered that we are thereby limiting the amount of superheat 

 that may have been acquired as a result of those processes attendant 

 upon the rise of the magma and proportional in amount to the extent 

 of rise. 



It is plain that there can seldom be more than a very small 

 amount of superheat left after the heating up of immersed frag- 

 ments. Occasionally, perhaps, such small amounts may be avail- 

 able for the direct solution of fragments but it would require only 

 an excessively small amount of solution to use up this heat, a fact 

 that becomes apparent when the enormous discrepancy between the 

 specific heats of silicate liquids (0.2-0.3 cal.) and the solution heats 

 of silicates (50-125 cal.) is realized. 



We have thus deduced, from general considerations, that mag- 

 mas cannot be expected to have much superheat and in particular 

 that they cannot retain a significant amount after the immersion of 

 foreign fragments in such quantity that an important effect upon the 

 composition of the magma would ensue if solution did occur. This 

 deduction need not be regarded as contradictory to the observed 

 fact that on very rare occasions inclusions of foreign rock have been 

 found converted into glass by magmas and that the measured tem- 

 peratures of lavas at certain active volcanoes are sometimes high 

 enough to indicate a condition probably significantly above that 

 of beginning of crystallization. It is becoming increasingly appar- 

 ent that in central volcanoes there are sources of heat, probably 

 from exothermic gas reactions, that are capable of producing the 

 temperatures observed, but these must be regarded as locally con- 

 centrated where the gases have their vent. Moreover, inclusions 

 converted to glass are found almost exclusively in such extrusive 

 rocks. Even their rare appearance in intrusive masses need not 

 militate against the general conclusion reached, for, admitting a 



