JR. A. Daly — Mechanics of Igneous Intrusion. 37 



nearly all of the superheat of the basalt might be used in dis- 

 solving the gneiss. The total melting-heat of gneiss, if molten 

 at 1050° C, would be .about 400 calories. The heat-energy 

 required for the solution of one gram of the gneiss which has 

 an original temperature of 200° C. is (400 — 40 = ) 360 calories. 

 The heat-energy given off by one gram of basalt in cooling 

 from 1300° to "1050° C. is about (250 X "34=0 =) 85 calories. 

 One gram or mass-unit of gneiss would, then, be dissolved by 



( — -= ) 4'3 grams or mass-units of the primary basalt, pro- 

 vided all the thermal energy were used for solution. 



These various calculations are obviously very crude. They 

 take no account of conduction of heat away from the batho- 

 lithic mass, nor any account of possible exothermic or endo- 

 thermic chemical reactions between basalt and wall-rock ; nor 

 any account of the influence of water, chlorides, etc., derived 

 from the geosynclinal rocks which are assimilated.* These 

 substances held in the magmatic solution tend to lower the 

 solidification point of the syntectic. The result of the calcu- 

 lation would also be affected if we assume that the heavier 

 xenoliths. would sink to levels where the temperatures are 

 above 1300° C. Finally, the result would be different if we 

 postulate that the invaded formations, through the crushing 

 incident to orogenic movement before the intrusion, had been 

 heated above 200° C. Without here entering on the discus- 

 sion of these further complications, we may conclude that 

 probably from four to six volumes of the superheated primary 

 basalt would furnish the heat-energy necessary for the solution 

 of one volume of wall-rock. 



If this rough estimate is even approximately correct, w T e 

 have some idea of the actual assimilating power of plutonic 

 magma which has been superheated a couple of hundred 

 degrees. We also see a definite reason for the fact that post- 

 Archean granites have never, so far as known, stoped their 

 way to the earth's surface. The crust has been too thick, the 

 expenditure of heat-energy in forming the syntectic magma 

 too vast, that the process could operate to its extreme and so 

 endanger the stability of the crust-roof above each batholith. 



Objection fou?ided on rarity of evidences of assimilation at 

 observed wall-rocks. — One of the most commonly expressed 

 objections to any theory of the replacement of invaded for- 



* According to the stoping hypothesis almost all of the heat conducted 

 into the shells of country-rock successively stoped away during the mag- 

 matic period, is not lost, hut is available for the abyssal assimilation of the 

 engulfed blocks. In view of the slowness with which the mixtures of pow- 

 dered silicates melt, it is probable that notable exothermic reactions do not 

 take place. The possibility of endothermic reactions seems to be a more 

 open question. 



