162 



On account of the viscosity the actual size of the crystals 

 would be small, for diffusion currents would not move 

 sufficiently rapidly to supply the growing crystal. 



The conditions above described probably represent the 

 "labile" state of undercooling, as described by Ostwald and 

 Miers.( 15) The micropegmatite, on this view, represents the 

 composition of the hypertectic rather than the eutectic point. 



Near the summit of the fissure chamber, crystallization 

 would be initiated, as this is the point of maximum under- 

 cooling due to the combined effect's of cooling and of 

 diminished volatile components in the liquid magma. 



Concurrently with the initiation of crystallization, at 

 the top of the fissure chamber, the reduction in pressure of 

 the volatile phase would initiate the formation of bubbles of 

 gas or vapour, predominantly, water dissolved in the liquid 

 magma and other volatile mineralizers, among wmich were 

 compounds of boron (boric acid). These would originate 

 throughout the depth of the fissure chamber and, viscosity 

 permitting, would gradually rise in the magma chamber, 

 enlarging both by reduction in pressure during upward move- 

 ment and, possibly, by coalescence of two or more bubbles. 

 At this stage the magma chamber is pictured as filled with a 

 more or less viscous silicate liquid, crystallization having 

 developed at its summit and, forming a network of crystals, 

 gradually extending downwards, and, at the same time, 

 ascending bubbles of gases (mineralizers) present in its lower 

 layers. Witili the removal of anhydrous minerals at the 

 crystallization level, additional gases would probably be set 

 free. The bubbles, in ascension, on reaching the network of 

 solid crystals of quartz and felspar would attach themselves 

 to these in the form of bubbles. The gases released on 

 crystallization would do likewise. 



The fissure magma is now pictured as a partially fluid 

 mass containing a network of crystal silicates, some of which 

 are enveloped in bubbles of the gas phase. It is probable 

 that these might occupy definite restricted horizons of the 

 fissure chamber. The volatile, components present in the gas 

 phase are assumed to have been, predominantly, w T ater and 

 boric acid. 



With a further reduction in temperature these mineral- 

 izers take up an active role and enter upon a destructive 

 stage. The felspar becomes unstable, and in an interaction 

 with boron compounds tourmaline is produced in situ, the 

 felspar being partially or completely replaced, according to 



(15) Vide the numerous papers by Miers and his co-workers. 

 References to these are quoted by A. Harker, Natural History 

 of Igneous Rocks, p. 208. 



