﻿312 THE ITT EAETH-MOTEITEXIS OF COLONS AY. [Mav 1908, 



injected in the intervals between separate movenients or separate 

 phases of a great epoch of morement. 



The Peesidext remarked on the importance of the definite 

 ascertainment of distinct periods of earth-movement in the tectonic 

 construction of the crystalline rocks of the Scottish Highlands. He 

 was personally conversant with the grounds on which a Torridonian 

 age had been assigned to certain rocks on the west coast of Scotland. 

 But it was only an hypothesis, highly probable, though not yet, so 

 far as he knew, proved to be true. The Author had spoken of the 

 Colonsay rocks as Torridonian, Avithont further comment ; and the 

 President asked whether any more definite indication of their age 

 had been ascertained than was available when the ground in Islay, 

 lona, and the west of Mull was examined. With regard to the age 

 of the later earth-movements in the Highlands, to which Dr. Pletfc 

 had alluded, it had long been recognized that some of them were 

 not older than the early part of the Old-Eed-Sandstone Period. 

 The President had even connected them with superficial changes 

 giving rise to the basins in which a large part of the Lower Old 

 Eed Sandstone was accumulated. 



The ArxHOE, in reply to the President's enquiry regarding the 

 presumption as to the Torridoniau age of the sediments, referred to 

 the correlation made by Dr. Peach in the recently issued memoir on 

 the Geology of Islay, and stated that the comparison of Colonsay 

 with the Ehinns of Islay, made recently by Mr. Bailey and himself, 

 had tended to confirm that correlation in so far as it placed the 

 sedimentary rocks of both districts in the same great group. With 

 regard to a possible correlation with the Diabeg Group of Skye, the 

 Author could not speak from his own experience, but he had had 

 the advantage of the opinion of Mr. Clough, who originally mapped 

 the latter district, and, after seeing the Colonsay succession, con- 

 sidered that the similarity of types in the two areas was sufficient 

 to justify the assumption that they were dealing with a portion 

 of the same general sequence. The relation to the gneiss in the 

 northern end of Colonsay was not sufficiently clear to be regarded 

 as evidence of the Torridonian age of the sediments. 



The Author had refrained from making any correlation with 

 the folding in other districts, feeling that such generalization was 

 premature in the present state of our knowledge. Mr. Clough had 

 suggested the possibility of a movement in the district of Cowal 

 prior to that which produced the foliation of the Dunoon phyllites. 

 If an analogous movement had occurred in Colonsay, its results had 

 been most effectually cloaked by the later movements. The term 

 lamprophyre had certainly been used in a very wide sense in 

 the paper. The advanced decomposition of the dykes and sills 

 intruded before the second movement, was in favour of the idea that 

 they belonged to the lamprophyre group. Those later than the 

 second movement were typical vogesites. The Author referred 

 again to his indebtedness to Mr. Bailey for help and advice, both 

 in the field and in the writing of the paper. 



