bowen: significance of glass-making processes 93 



separation into two liquid portions, the lighter of which contains 

 most of the water together with much silica, alumina, and 

 the alkalies.^ This is, of course, a possibility not altogether to 

 be excluded, nevertheless all the available evidence is against it. 

 Such experimental work as has been done hitherto on aqueous 

 silicate melts gives no indication of a tendency towards a separa- 

 tion into two liquid layers.^ But the range of this work is limited 

 as yet and one must fall back largely upon examination of the 

 geological evidence. Over against the lack of types intermedi- 

 ate between gabbro and granite (granophyre, micropegmatite) 

 in some localities should be placed the abundance of intermediate 

 types elsewhere. Again, if we examine the gabbro of a gabbro- 

 granophyre occurrence we almost invariably find a certain 

 amount of the granophyre occurring in the gabbro, frozen in as 

 Daly puts it.^ And when we examine the manner of occurrence of 

 this frozen-in material we find nothing to lead us to believe that 

 it represents an immiscible liquid. It does not form globular 

 masses, large and small, scattered through the gabbro. It occu- 

 pies crj^stallization interstices with all the marks of a crystalliza- 

 tion residuum. Add to this the fact that it corresponds in com- 

 position with the kind of crystallization-residuum one is led to 

 expect from experimental studies, and the reasons for appealing to 

 liquid immiscibility may be regarded as of insignificant weight. 

 Many petrologists regard liquid immiscibility as the ready solu- 

 tion of all difficulties. Realizing that present evidence is against 

 it, some are led to ''hope" that it may yet be experimentally 

 demonstrated in silicate magmas. Until then one must re- 

 gard its occurrence in silicate magmas as resting on pure assump- 

 tion, an assumption that is in most cases not even helpful, and 

 probably never preferable to the well-supported theory of 

 crystallization-differentiation.'' 



' J. W. Evans. Discussion of paper by G. W. Tyrrell on The picrite-tes- 

 chenite sill of Lugar. Quart. Journ. Geol. Soc. 72: 130. 1917. 



■* G. W. MoREY and C. N. Fenner. The ternary system H20-K2SiOi-Si02. 

 Journ. Am. Chem. Soc. 39: 1173. 1917. 



5 Igneous Rocks and Their Origin, p. 241. 



^ I would be the last, however, to claim complete miscibility between sul- 

 fides and silicates. Cf. Tolman a\d Rogers {A study of the magmatic sulfid 

 ores. Stanford Univ. Publ. 1916, p. 10). 



