260 XEirOLTTHIC MINOR I1S-TEI7SI0NS IN MULL. [vol. Ixxviii^ 



those characterizing the higher thermal zones of the Archaean 

 rocks, and occur continuously through belts of considerable 

 breadth and of unknown length. The Author's work thus sup- 

 ports the view now generally entertained that the distinctive 

 minerals in the Archaean rocks are due to deep-seated changes. 



Dr. J. W. Etajs^^s remarked on the number of important facts 

 disclosed by the Author's communication, and the interestino- 

 deductions which he drew from them. It was remarkable that an 

 argillaceous rock should contain as much soda as that found in 

 the buchite, and that the ratio of potash to soda should be 

 increased by the metamorphism due to an igneous rock con- 

 taining more soda than potash. He thought that the changes 

 described might have taken place at a comparatively-moderate 

 depth. 



The Author expressed his gratification at the reception ac- 

 corded to his paper, and thanked the speakers, especially Sir Jethro 

 Teall, for their kind remarks. As pointed out by Mr. Barrow, 

 he considered himself extremely fortunate in having such beautiful 

 material to work upon. In reply to Dr. Evans he admitted the 

 somewhat unusual alkali-ratio presented by the buchite ; but 

 analyses of aluminous sediments show that an excess of soda 

 over potash is occasionally met with, and he would rather regard 

 this as an inherent property of the shale than as a result of 

 metamorphism. With respect to the depth at which the meta- 

 morphism of the xenoliths was accomplished, all that could be 

 said was that it took place within the magma-basin and under 

 a pressure sufficient to prevent the expulsion of any constituents 

 in a gaseous form. 



