754 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



would be about 400 calories. The beat 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 (250x-340=) 85 calories. One gram or mass-unit 



of gneiss would, then, be dissolved by ( ^-= ) 4-3 grams or mass-units of 



the primary basalt, provided 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 batholithic mass, nor any account of 

 possible exothermic or endothermic chemical reactions between basalt and wall 

 rock; nor any account of the influence of water, chlorides, etc., derived from any 

 geosynclinal rocks which are assimilated.* These substances held in the mag- 

 matic solution tend to lower the solidification point of the syntectic. The result 

 of the calculation 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 discussion of these further com- 

 plications, 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, we 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 seldom, if ever, stopecl their wfty to the earth's 

 surface. The crust has been too thick, the expenditure of heat energy in form- 

 ing the syntectic magma too great, that the process could operate to its extreme 

 and so endanger the stability of the roofs of most of these batholiths. 



Objection Founded 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 formations by batholithic magmas consists in emphasiz- 

 ing the obvious fact that the average xenolith and average wall-rock of batholiths 

 do not show direct evidence of melting or of solution in the granitic magma. 

 This objection has been answered by the writer in several publicationsf and 

 also by Andrews in most vivid fashion.:}: The point has, however, been restated 

 by several authorities without any adequate discussion of the subject. No one 

 can deny that, when the magma is all but frozen, it is incapable of assimilating 



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

 shells of country-rock successively stoped away during the magmatic period, is not 

 lost, but is available for the abyssal assimilation of the engulfed blocks. In view of 

 the slowness with which the mixtures of powdered silicates melt, it is probable that 

 notable exothermic reactions do not take place. The possibility of endothermic re- 

 actions' seems to be a more open question. 



t Amer. Jour. Science, Vol. 15, 1903, p. 281 ; Bull. Geol. Soc. America, Vol. 17, 1906, 

 p. 372. 



J E. C. Andrews, Records, Geol. Surv. of N. S. Wales, Vol. 8, Pt. 1, 1905, p. 126. 



