398 N. L. BOWEN 



magmas but of normal uncontaminated magmas. Daly considers 

 differentiation as particularly prone to occur in syntectic magma 

 because "equilibrium is upset" by the foreign material. The 

 writer, on the other hand, gives his reasons for believing that the 

 chilling effect normally consequent upon the immersion and solution 

 of foreign material in a magma is sufficient to limit very materially 

 the differentiation of magma that has been so affected,' and thus 

 arrives at substantial agreement in this respect with Harker when 

 he makes the statement, ''Hybrid rocks are barren." In this 

 connection it seems desirable to consider a statement made by 

 Shand, to which Daly refers with approval.^ The statement is 

 that foyaitic or phonolitic magmas may enter "into chemical 

 combination with the silica of invaded rock masses. The reactions 

 thereby induced would he exothermic and would tend to raise the temper- 

 ature of the magma. The access of heat produced in this way would 

 in turn enable the magma to perform a further amount of work in the 

 way of mechanical solution.''^ In short, one might say that the 

 addition of solid rock to a magma is, in this case and possibly in 

 others, a mere adding of fuel to the flames. The conception prob- 

 ably had its origin in the long-used terms "acid" and "basic" 

 as applied to igneous rocks, and in an unconscious extension of 

 the analogy implied in these terms. It is true that when an acid is 

 added to a base they unite with avidity, so much so that one 

 must make the addition with care on account of the consequent 

 rise in temperature. No such care is necessary when silica is 

 added to molten nephelite.'' Even at a temperature of 1550° C, 

 where the product of the reaction (albite) is superheated over 

 400° C, silica dissolves in nephelite with excessive reluctance. 

 Indeed experimental study suggests that the following general 

 equation could be written with some confidence: molten rock+ 

 solid rock = molten rock— :i; cal. It is not improbable, however, 

 that, in some cases at least, the following equation would hold: 

 molten rock+molten rock = molten rock+o; cal. In other words, 



^ "The Later Stages of the Evolution of the Igneous Rocks," Jour. Geol., XXIII 

 (191 5), Supplement, pp. 85-86. 



^ Jour. Geol., XXVI (1918), no. ^ Italics are mine. 



4 N. L. Bowen, "The Composition of Nephelite," Amer. Jour. Sci., XXXIII 

 (1912), so. 



