308 ON SOME ADDITIONAL OCCURKEXCES OF TACHTLYTE. 



older tuffs, but the latter were generally of a more acid character. 

 The specimen of variolite was unusual. This rock occurs in situ 

 on the east side of the Durance, near Briancon, in thin dykes. 



Col. McMahon also expressed his gratification at hearing the 

 paper. 



Prof. JuDD was pleased to think that he had suggested the study 

 of this class of rocks to IsLt. Cole. There were, in the paper, many 

 interesting points of comparison between the basic and the better- 

 known acid glasses. 



llr. Teall observed that there was very little to criticize in the 

 paper. He spoke of the interesting generalization to which the 

 Author made his facts point. 



Mr. HuTLET said that the remarkable opacity of this group of 

 rocks, due to disseminated magnetite, often rendered microscopic 

 observations very difficult. He contrasted the structure of the 

 Ardtun rock with that of certain artificially devitrified glasses. 

 It would be interesting to ascertain the relative specific gravity 

 of the spherulitic and non-spherulitic portions. He had seen 

 structure somewhat similar to that of the Ardtun specimen in rocks 

 of the same class from one or two other localities. 



The Atjthoe thanked the speakers for their kind reception of his 

 paper. He behoved that the occurrence of tachylyte-fragmeuts 

 in the old rocks was no new point. The observation of perlitic 

 structure in variolite was due to IIM. Eouque and Ijevj. To 

 Mr. Eutley he replied that spherulites, apparently similar, occur in 

 tachylytes of different compositions. The question of the specific 

 gravity of these bodies presents considerable difficulties. 



