﻿vol. 66.] schists op the scottish highlands. 610 



Discussion. 



Prof. Sollas welcomed this as an important contribution to our 

 knowledge of mountain-formation. It was a masterly paper, in 

 which a very complicated structure had been explained in a clear 

 and convincing manner. The task of establishing a sequence 

 among highly disturbed rocks, not containing fossils, might well 

 have deterred a less courageous investigator ; but, as the Author 

 unfolded the steps of his argument, they saw difficulty after difficulty 

 disappear, and witnessed at length the final triumph of order over 

 chaos. The only undetermined point was whether the sequence 

 was normal or inverted, and in leaving this question undiscussed the 

 Author had shown a wise reserve. The recumbent sheets and folds 

 showed close analogies with many of those which had been described 

 in Switzerland. Among the more interesting phenomena was the 

 passage of folding movements into flows. In considering the flow 

 of solid rocks, it was important to bear in mind not only the element 

 of time, but also that of space. It was impossible to argue, from 

 the physical constants of a rock examined in a hand-specimen, to 

 those of the same rock buried deep within the crust and forming 

 part of a flake 12 miles long or more. 



The granitic intrusions, as the Author remarked, had no relation 

 to the folding movement ; but the granite before its intrusion may 

 have played an important part : when the final attack is made on 

 the explanation of mountain-formation, this will have to be taken 

 into account. 



Dr. Teall regretted the absence of the Author's colleagues, whose 

 views on the paper would have been welcome. He believed that 

 they were of opinion that the paper represented an important 

 advance in our knowledge of the tectonics of the Central Highlands. 

 It certainly introduced new ideas which were deserving of serious 

 consideration. 



Mr. M. M. Allorge commented upon the remarkable geometrical 

 power shown by the Author in unravelling the complicated struc- 

 tures of this district, and grasping at the same time the strike, the 

 dip, and the superimposition of the successive recumbent sheets, 

 notwithstanding the partial obliteration produced by subsequent 

 granitic intrusions ; it was like reading a palimpsest half obscured 

 by writing of a later date. The squeezing of the cores of some of 

 the recumbent sheets and the bifurcating crystalline tongues thus 

 formed reminded one of the ' carapaces ' described by M. Argand 

 in the crystalline zone of the Alps near Zermatt. 



Any additional information concerning the following points 

 would be gratefully welcomed: — (1) Were the thrust-planes under- 

 lain by thrust-breccias or not ? (2) After the general ' mise en 

 place ' of the sheets, had they been subjected to one or to several 

 subsequent epeirogenic movements? (3) Was there any infor- 

 mation available concerning the amount of denudation undergone 

 by these mountain -structures, and consequently the depth at which 

 they originated ? 



Q. J. G. S. No. 264. 2 u 



