770 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



2. Limited Misctbility. — Ostwald points out that the number of liquids 

 iniscible only within definite limits is much greater than is the number of 

 those which mix in all proportions." Since magmas are solutions it is a priori 

 wise to consider their possible differentiation through the principle of limited 

 miscibility at certain temperatures. Though Vogt has held that this principle 

 does not, in general, apply to silicate mixtures, one of his latest publications 

 contains the statement that, while separation of minerals follows the eutectic 

 law, the ' mineral in excess ' separates out while still in the liquid phase. f He 

 further holds that magmatic differentiation is chiefly the result of just this 

 kind of separation. Unless the writer misapprehends his meaning, Yogt has 

 come to recognise limited miscibility as a general law for silicate solutions so 

 soon as these approach the consolidation point of temperature. Ostwald and 

 Richards believe that crystals develop from a transitory liquid phase in the case 

 of substances which melt at temperatures not far from the temperature of 

 crystallization.^: 



From Durocher's time. to the present many investigators have agreed in 

 favour of limited miscibility of components in molten magmas. In view of the 

 great difficulties surrounding experiments with molten silicates, including the 

 granting of sufficient duration to an experiment, he is a bold physical chemist 

 who denies the possibility of the separation of liquid components through the 

 entrance of immiscibility at certain temperatures (and pressures). Without 

 pursuing this theme in chemical dynamics it suffices to point out that the actual 

 rocks in nature show unequivocally that separation by limited miscibility, a 

 true magmatic splitting, has taken place, often on a great scale. This is true of 

 silicate magmas splitting from silicate magmas as it is true of sulphidic or 

 metallic melts separating from silicate magmas. 



Most basic segregations and probably all orbicular granites, diorites, and 

 gabbros are direct evidences of the emulsion stage, which precedes the separation 

 of immiscible liquids with fall of temperature. The common banding of 

 nephelite syenites, the banding of certain gabbros, the phenomena of some 

 differentiated dikes (Entmischte Gange), are other illustrations of this splitting 

 in liquid magmas. The constitution of the Moyie sills or of the Sudbury sheet is 

 inexplicable except on the assumption of immiscibility of granitic (micropeg- 

 matitic) and basaltic (gabbroid or noritic) magma under certain conditions. 

 (See chapter X). Backstrom's point that there is a lack of intermediate 

 rocks in the liparite-basalt field of Iceland has great significance in this con- 

 nection^ Finally, the evidence of silicate melts in glass factories is conclusive 

 as to the main principle. 



No one will, of course, deny that silicate melts are miscible in all proportions 

 at high enough temperatures. The question is as to whether the average 

 magma tends to assume the emulsion state within, say, one or two hundred 



* W. Ostwald, Solutions, 1891, p. 39. 



t J. H. L. Vogt, Videnskabs-Selskabets Skrifter, I, Math.-Naturv. Klasse, No. 10, 

 Christiiania, 1908, pp. 6, 16, and 102. 



tT. W. Richards. Philosophical Magazine, 1901, p. 500. 

 § H. Backstrom, Jour. Geol., Vol. 1, 1895, p. 773. 



