558 kentallentte and other argyllshire rocks. [aug. i9oo, 



Discussion. 



The President said that he had a personal interest in this type 

 of rock. He had described a specimen from Kentallen in ' British 

 Petrography' as a plagiocl a se-augite- olivine -mica 

 rock. Later on he had re-examined the rock for the Geological 

 Survey, and had recognized orthoclase as an important constituent 

 of certain varieties. He had felt that the rock was peculiar, but 

 in the absence of full knowledge as to its field-relations had' 

 hesitated to invent a new name, and so had termed it olivine- 

 monzonite. The Authors had worked out the field-relations of 

 the rock, and had examined many varieties. He thought that they 

 were justified in proposing a new name. Apart from general ap- 

 pearance and mode of weatheriDg, both of which were characteristic, 

 the distinguishing feature was the abundance of olivine and augite 

 in association with varying proportions of orthoclase and plagioclase. 

 If Prof. Brogger's definition of monzonite were accepted, it could 

 not be applied to this rock. Some varieties were rich in ortho- 

 clase. This indicated affinities with olivine-minettes and with some 

 of the Permian traps of the Exeter district, in which Prof. Watts 

 had detected olivine and orthoclase. Basic rocks rich in potash 

 also had chemical affinities with leucite-bearing lavas, and he felt 

 that if leucite were ever found in England it would be in the 

 Exeter district. 



Sir Archibald Geikie said that he wished to express the gratifi- 

 cation with which he welcomed so careful a paper by two members 

 of his Survey staff. The field-relations of a remarkable group of 

 rocks had been worked out with great skill, and a fresh example had 

 been studied and illustrated of the connexion of a granite-core with 

 peripheral basic extrusions. Though we were still far from having 

 solved the mysteries of 4 differentiation,' there could be no doubt 

 that their solution would be most satisfactorily and speedily reached 

 by such a research as that which had been brought forward by the 

 Authors, combining, as it did, an ample collection of field-evidence 

 with a minute study of the internal composition and structure of 

 the rocks. 



Prof. Sollas commented on the interesting nature of the rock 

 which had been so excellently described and illustrated. The 

 collection of more basic material in the cooler portions of a magma 

 offered a tempting basis for a consistent hypothesis of differentiation; 

 but it seemed still doubtful whether this hypothesis deserved the 

 importance that it had acquired. Marginal changes in dykes might 

 be produced in more ways than one, and the appearance of quartz 

 in olivine-beariug rocks was suggestive of the action of water. 

 Terms which could only be spoken of as barbarous — such as 

 facies-suit — by no means conduced to clearness of expression, 

 and it was to be hoped would never find a home in the English 

 tongue ; nor did it appear that anything was gained by framing a 

 new substantive name from Kentallen, which might with greater 

 advantage have been used as an adjectival qualification. 



Prof. Watts and Mr. Prior also spoke. 



