Vol. 70.] INTRUSION'S NEAR MELROSE. 315 



Discussion. 



Prof. W. W. Watts remarked on the peculiarity of the soda- 

 bearing rocks described bj the Author, in that they carried rie- 

 beckite and aegirine, but apparently no nepheline. He asked how 

 far the connexion shown in the section between dykes and sills Mas 

 supported by field observations. 



Mr. W. Campbell Smith commented on the very wide sense in 

 which the term ' trachyte ' was used in the paper. He thought 

 that many of the rocks mentioned were equivalent to those described 

 by Dr. Prior as 'phonolitic trachytes,' while others containing 

 more segirine-augite and olivine might be acid members of the 



* trachydolerite ' group. Dr. Prior had described closely similar 

 rocks from British East Africa and from the Antarctic. 



He asked whether it was possible to determine the exact nature 

 of the olivine in these rocks. The presence of fayalite in relatively- 

 acid rocks, rich in iron-oxides and poor in magnesia, had been 

 recorded recently, and further records of this mineral would be 

 interesting. 



Dr. J. V. Elsden said that particular interest attached to the 

 rock described by the Author as olivine-trachyte, which appeared 

 to be a highly specialized type and one that, if consolidated under 

 plutonic conditions, might have been expected to produce a rock 

 something like kentallenite. He regretted the necessity for apply- 

 ing the term ' trachyte ' to these rocks, all of them being intrusives. 

 He congratulated the Author upon her careful and detailed work 

 in this area, Avhich in some respects recalled certain features of the 

 Chris tiania district. 



Dr. J. W. Evans referred to the different use of the term 



* trachyte ' in Germany and in the United Kingdom. In Germany 

 the essential difference between a trachyte and an orthoclase- 

 porphyry or orthophyre was formerly one of geological age and 

 was now one of degree of alteration, and there was no objection to 

 applying the former term to an intrusive rock. On the other hand, 

 although a few British authors made the structure the criterion, as 

 the Author had done, the great majority undoubtedly restricted its 

 use to volcanic rocks. He was, however, doubtful whether such a 

 restriction could be losricallv defended. 



The Author, in reply, indicated on the sections those portions 

 of the area that could be mapped with accuracy, arid those that 

 were largely covered with screes. She had detected a mineral 

 resembling fayalite in some of the acid trachytes : it forms small 

 spherulitic growths with glass as interstitial matter. She added 

 that petrographical nomenclature was always a most difficult 

 matter, and presented ample scope for differences of opinion ; but 

 ^he had tried to do the best that she could with existing rock- 

 names. In conclusion, she thanked the Fellows present for the 

 kind reception given to her paper. 



