160 



these nodules were not observe! in the coarser-grained granite; 

 they appear to be confined to the finer-grained varieties" ; 

 and again, ( g ) "Their home is in the fine-grained tourmaline 

 granite and the white granite, and it is in the former that 

 they undoubtedly reach their maximum development." 



The author, after consideration of the occurrence at 

 Cape Willoughby, suggests that the following processes have 

 co-operated in their production: — 



The crystallization of the main mass of granite was 

 followed by the production of fissures and joints as a result 

 of contraction through cooling. Into these fissures was in- 

 jected the still liquid residue of the magma enriched in 

 mineralizers, and forming what are now the aplites. The 

 sudden injection of a highly fluid mass charged with volatile 

 products, primarily water with other mineralizers, would, the 

 fissure being spacious enough, provide an avenue of temporary 

 escape for the more volatile products. A magma of this 

 nature is thus characterized by a remarkable mobility of 

 equilibrium. It is possible that, at this stage, the principal 

 mineralizer, water, was present, partly as a gas, below its 

 critical temperature. ( 10 ) 



The increased magma space thus originating through 

 Assuring, the resultant effects are : — 



(1) A reduction of pressure due to expansion of the 

 gas phase; concurrently a reduction of the con- 

 centration of the volatile components in the liquid 

 magma. 



(2) An increase in the viscosity of the silicate liquid 



due to reduction of the active mass of the mineral- 

 'izers in the liquid. 



(3) Increasing the crystallization temperatures or 

 freezing points of the silicates in solution. 



[It is to be noted here that a lowering of freezing point 

 generally accompanies a reduction of pressure, but this effect 

 must be enormously outweighed by the decreased fusibility 

 (solubility) consequent on diminished concentration of vola- 

 tile components. This latter effect apears to have been 

 ignored or denied by some petrologists, e.g., Schweig (n ) even 



(9) Loc. cit., No. 21, p. 73. 



(10) It does not follow from this that the temperature was 

 below 370° C. — the critical temperature for pure water. A gas 

 dissolved in a mixture of non-volatile components has a higher 

 critical temperature than it possesses in the pure state. This 

 elevation of the critical temperature is analogous to the elevation 

 of boiling point by dissolved substances. 



(ii) M. Schweig: Neues Jahrbuch Beil., Bd. 17, 1903, p. 516, 

 et. seq. 



