M. A. Daly — Mechanics of Igneous Intrusion. 35 



These three sources of superheat would alone furnish enough 

 thermal energy to raise the injected basaltic magma from 1140° 

 C. to some temperature short of 1500° C. or 1600° C. 



The piling up of 10,000 meters of lava over a large area 

 would have an analogous superheating effect on the substratum. 

 This conclusion enables us to give some explanation of the fact 

 that the lavas of Kilauea and Mauna Loa seem to be the 

 hottest known in any volcanic vent. The vast Hawaiian lava- 

 plateau has, apparently, been built up by the comparatively 

 rapid effusion of basaltic flows from Pacific depths averaging 

 6,000 meters to heights above sea of about 4,000 meters. The 

 unique lava-fountains of the calderas, while showing obvious 

 evidence of considerable su perfusion, are described as glowing 

 with " white heat." * If a correct description, this implies a 

 temperature of 1300° C. or possibly 1400° C.f Such temper- 

 ature must be a minimum for the substratum which feeds the 

 calderas, where there is continuous loss of heat in the convec- 

 tively stirred lava. 



Speculative argument and limited observations in nature 

 agree, then, in fixing some such temperature as 1300° C. as a 

 minimum for the basaltic mass injected into the crust-rock 

 below a great mountain range. 



Capacity of superheated, plutonic magma for melting and 

 dissolving xenoliths. — Basalt must have a thermal capacity 

 much like that of diabase at the same temperature. Barus's 

 experiments show that the average specific heat of diabase for 

 the interval 1300-1140° C. is *350. % The heat-energy contained 

 in the substratum, if it be superheated 160° C. above its 

 melting-point (1140° C), is in excess of that contained in the 

 substratum just above its melting point by (160 X '350 = ) 55 -f 

 gram-calories. 



This surplus heat-energy is available for the fusion and 

 assimilation of country-rock. There are good reasons for 

 believing that the average wall -rock of granite batholiths has 

 the composition and crystallinity of a granitoid gneiss. For 

 purposes of calculation this will be assumed to be the fact. 

 The average temperature of the wall-rock before an abyssal 

 intrusion may be conservatively estimated from the normal 

 temperature gradient to be 200° C. In order to raise the 

 gneiss to the temperature of 1200°, where it is just molten, 



* J. D. Dana, Characteristics of Volcanoes ; New York, 1891, p. 200. 



f LeChatelier and Boudouard's High Temperature Measurements ; New 

 York, 1904, p. 246. 



X C. Barus, op. cit., p. 53. For the interval 100-20° C. the mean specific 

 heat is about "185. There is, in fact, a steady increase in the mean value 

 as the temperature of any silicate or silicate mixture rises. This fact goes 

 far to explain the prolonged liquidity of assimilating magmas. Cf . J. H. L. 

 Vogt in Christiania Videnskabs-Selskabets Skrifter, math-naturv. Klasse, 

 1904, No. 1, p. 40. 



