BY LEO A. COTTON. 227 



intruding solution. The curves indicate a minimum SiO., per- 

 centage in the neighbourhood of the contact of vein-material and 

 oouu try- rock. 



From the numerous liquid inclusions present in the quartz of 

 the vein-material, it is evident that the conditions of introduction 

 of the vein-solutions must have been such that the pressure and 

 temperature were above those at the critical point.* 



In endeavouring to ascertain the nature of the intruding rock 

 solution several principles seem to me to be necessary. These 

 are : — 



(1) Those constituents characteristic of the intruding solution 

 should show a progressive decrease from the centre to the margin. 

 Due allowance must, however, be made for magmatic differen- 

 tiation within the vein itself. 



(2) Those constituents characteristic of the country-rock and 

 not of the vein, should show a decrease from the margin to the 

 centre of the vein. 



(3) Those constituents formed by the action of the intruding 

 solution on the ' country-rock should show a decrease on either 

 side of the zone of maximum metasomatic activity. 



(4) Constituents which occur in approximately equal amounts 

 in the primary vein-solution and the country-rock should appear 

 fairly uniformly distributed throughout the whole replacement- 

 vein. 



(5) That, besides metasomatic activity in sitic, evidence of 

 metasomatism with transportation of substance should be sought 

 for. 



The processes active in the vein between A and C seem to 

 have been dominated by magmatic differentiation of the intruding 

 pneumatolytic solution. This differentiation appears to have 

 been effective in concentrating the siliceous material chiefly at 

 the centre, and the ferromagnesian constituents towards the 

 margin. Between C and E the ferromagnesian constituents 



* The critical temperature for water is 365° C at a pressure of 200 

 Atmospheres. 



