Vol. 65.] POEPHYEITIC LABRADORITE-CEYSTALS. 103 



separation was essential to reduce the residue to an eutectie mixture 

 of sufficiently low consolidation-point to enable it (under this 

 pressure) to remain liquid at all. 



Dr. J. "W. Evans thought that there was every reason to believe 

 that, as the previous speaker had suggested, the magnetite had 

 crystallized out before the plagioclase. The Author contended 

 that the plagioclase continued to separate out — with increasing 

 acidity' — till there was just sufficient felspathic material left to 

 form an eutectie mixture with the pyroxene. If he were right, 

 the interesting conclusion followed that the acidity of the plagioclase 

 of the ground-mass depended on the amount of pyroxene present. 

 The speaker believed that a long series of careful studies like the 

 present would be required, before the laws that governed the 

 crystallization of minerals from rock-magmas could be worked out. 



The President (Prof. Sollas) remarked that the formal thanks 

 of the Society had been emphasized by the warm welcome with 

 which this communication from their esteemed Foreign Member 

 had been received. The lucid exposition by Prof. "Watts gave a full 

 account of the investigation. It was evident that the phase-rule 

 was a powerful instrument, which in skilled hands was capable of 

 successful application to many of the deeper problems of petrography. 

 That the simpler problems should be attempted first was natural ; 

 more complicated and anomalous cases would be attacked later. 

 The notion of eutectics, introduced by Dr. Teall, was sound, and its 

 study was becoming increasingly fruitful. 



Prof. Watts, replying on behalf of the Author, apologized that 

 he had only been able to give an imperfect account of a very 

 lucid paper, the outcome of a great deal of chemical and micro- 

 scopical work. He stated that the Author had deliberately 

 excluded questions of supersaturation from consideration, and had 

 reduced the eighteen or more actual components of the rock to a 

 minimum of three components or groups of components. 



