J. V. Elsden— Origin of Pegmatite Veins. 313 



hornblende, biotite, plagioclase, some orthoclase, and a little quartz. 

 No microcline was observed in the slices examined. These dark 

 segregations, therefore, differ essentially from the normal rock, and 

 may be safely regarded as a result of the first phase of consolidation. 

 The deformation of these basic secretions is in accord with previous 

 observations in other localities,^ and would be expected to occur 

 under the influence of magmatic movements. Although these dark 

 patches are usually regarded as true results of magmatic segregation, 

 the primdren ausscheidungsschlieren of Zirkel, it may be mentioned 

 that Eeyer and others regard even these as having resulted from an 

 original imperfect mixing of a more or less heterogeneous magma. 

 The junction between them and the normal rock is well defined, and 

 in a slide showing the contact the transition is sudden from the 

 diorite to the microcline granitite. 



Passing now to the pegmatites, these are very coarsely crystalline 

 mixtures of large microcline crystals and quartz, representing the 

 pegmatite of Haiiy, although there is not any conspicuous orientation 

 of the quartz. Accessory minerals are not altogether wanting, and 

 in some examples the pneumatolytic species seem to be more 

 abundant than in the parent rock, a feature which was also noticed 

 by Brogger in the Christiauia district. 



It is obviously conceivable that we might consider these veins 

 to be a product of magmatic differentiation, which began with the 

 consolidation of the dioritic patches, and concluded with the streaks 

 and veins of pegmatite, something after the nature of the streaks 

 -of low freezing eutectic observed in some alloys. But it would be 

 difficult on this assumption to account for the regular, wavy con- 

 tortions, which could scarcely have been formed otherwise than 

 .in a still viscous mass. On the other hand, differentiation in a liquid 

 magma of such a nature as to develop such streaks is at least purely 

 conjectural. We do not yet know the limits of miscibility of 

 jsilicate magmas, nor whether it is possible for a binary mixture, 

 containing quartz and felspar, to segregate in distinct layers in 

 the midst of an ordinary ternary granitic magma. At the same 

 time, physical chemists are gradually extending our knowledge of 

 the series of liquids which only mix in all proportions between 

 certain well-defined limits of temperature,' as, for example, in the 

 case of molten zinc and lead. But in the present state of our 

 knowledge it would be unsafe to assume that a magma can become 

 streaky by mere hysterogenetic differentiation.^ 



But another explanation is possible. The magma may have been 

 subjected to an invasion by streams from contiguous areas. These 

 currents, if regarded as the acid residuum forced out from regions 

 where partial consolidation had already taken place, would be 

 differentiated as regards the liquid which they invaded, and might 



1 See Frosterus, "Uber einueuesVorkomnissvonKugelgranit," etc.: Tschermak's 

 Min. und Pet. Mittheilungen, xiii (1892), p. 177. 



' See Findlay, " The Phase Rule," chap, xiv (1904). 



2 Cf . Moroze\vicz, ' ' Experimentelle Untersuchungen liber die Bildung der Minerale 

 im Magma" : Tschermak's Min. und Pet. Mittheilungen, xviii, pp. 232-3. 



