444 DEVITRIFICATION IN GLASSY IGNEOUS ROCKS. [NOV. I903, 



to form such an association, and yet exclude other associations. 

 Quartz and an alkali-felspar very commonly occurred in suitable 

 proportions, as the Chairman had pointed out ; but conditions might 

 never arise such as would permit the entry of molecules of a third or 

 fourth mineral into the quartz-felspar associations, as the Authors 

 appeared to have suggested. 



Prof. Judd expressed his agreement with the many points which 

 had been brought out so clearly by the Authors, and concurred with 

 the Chairman in valuing the high importance of Messrs. Haycock & 

 XevilFs researches. Experimenters with alloys were able to regulate 

 at their pleasure temperatures and proportions ; on the other hand, 

 petrographers had the advantage of being able to use polarized 

 light. 



Prof. Sollas congratulated the Authors on their accurate and 

 logically - consecutive description of the phenomena. He thought 

 that the investigation of mixtures of transparent salts would afford 

 more valuable information than that obtained from the study of 

 alloys. With regard to the use of the term ' devitrification,' that 

 appeared to postulate the previous existence of glass, yet many of 

 the phenomena described might be due to direct crystallization 

 from a molten magma. 



Mr. Parkinson thanked the Fellows for the way in which his 

 remarks had been received. 



Prof. BoNNEir expressed his thanks to the Fellows of the Society 

 for their kindly reception of his paper. He was glad to use the 

 opportunity of acknowledging the help which he had received from 

 the Chairman's Presidential Address, and from the work on alloys 

 of Messrs. Stead, Haycock, Nevill, and others. He explained, in 

 reply to Prof. Cole, that he laid no stress on the possible presence 

 of a ferromagnesian mineral in an eutectic of a granitoid character. 

 Even without it we had an indeterminate equation of four variables. 

 In reply to Prof. Sollas, he said that Miss Paisin had formerly made 

 some experiments of crystallization in a colloid, but had not been 

 able to carry them far. He admitted that the term ' devitrification ' 

 had to be used rather vaguely, but thought that the inclusive sense 

 was defensible. 



