532 N. L. BOW EN 



rock-forming minerals are seldom so simple. Quartz is the only- 

 important example of a rock mineral of definite composition, prac- 

 tically all others being of a variable nature, that is, solid solutions. 

 The rock-forming pyroxenes do not fall behind in this respect and the 

 addition of pyroxene to a natural magma saturated with pyroxene 

 would in general be attended by reaction phenomena similar to those 

 we have described for the plagioclases. The precise nature of the 

 reactions in the case of the pyroxenes cannot be stated except for 

 the clino-enstatite-diopside solid solution series.^ 



In another paper the writer has discussed solid solution series and 

 offered reasons for calUng them continuous reaction series. The 

 importance of the reaction relation between liquid and crystals was 

 there discussed in its bearing on crystallization. Here we have seen 

 its importance in connection with the behavior of inclusions. 



As was pointed out in the paper referred to, there is another 

 type of reaction relation between liquid and crystals that is exhib- 

 ited in the reaction pair and the discontinuous reaction series.^ 



The existence of such series is again of great significance in 

 connection with the behavior of inclusions. An important reaction 

 pair are the oHvine, forsterite, and the pyroxene, clino-enstatite. 

 Their relation is exhibited in its simplest form in the binary system, 

 forsterite-sihca, of which the equilibrium diagram is shown in 

 Figure 7. The effect of adding crystals of the first member of a 

 reaction pair to a liquid saturated with the second member is well 

 illustrated by this system. A liquid of composition {M) is saturated 

 with clino-enstatite but lies on the unsaturated side of the metastable 

 prolongation of the forsterite liquidus. It is therefore unsaturated 

 with forsterite and we may imagine that around each added forster- 

 ite crystal a small quantity of liquid of composition {N) may form. 

 This condition is, however, metastable and from this liquid cHno- 

 enstatite would immediately be precipitated with formation of the 

 liquid {M). Through constant repetition of this formation from 

 forsterite of an infinitesimal quantity of the metastable liquid, 

 with immediate precipitation of clino-enstatite, the forsterite is 



' N. L. Bowen, Amer. Jour. Sci., Vol. XXXVIII (1914), pp. 228-37. 

 ^ "The Reaction Principle in Petrogenesis," Jour. GeoL, Vol. XXX (1922), pp. 

 177-98. 



