﻿Yol. 64.] THE TWO EAETH-MOVEMEISTTS OF COLONSAY, 311 



EXPLANATION OF PLATES XXXIII & XXXIV. 



Plate XXXIII. 



Horizontal surface of rock on the coast south of Cnoc Corr, at the north- 

 eastern end of Colonsay, showing sandy phyllite with the second cleavage 

 running in straight lines from left to i-ight across the picture, and crossing the 

 bedding. Traces of the first cleavage are recognizable in microscopic section, 

 but its relation to the bedding is not obvious. 



Plate XXXIV. 



[The slides are preserved in the collection of the Geological Survey.] 



Fig. 1. Phyllite (slide 13.524) on the shore near Cnoc Corr, in the north-eastern 

 end of Colonsay (Zone 4, fig. 2, p. 301), showing the first cleavage 

 crossing the figure from left to right, with quartz-veins segregated 

 along it. The second cleavage, an alternating slip-cleavage, is seen 

 crossing the first almost at right angles, in a direction parallel to the 

 axial planes of the folding by which both the quartz- veins and the 

 first cleavage have been affected. ( X 18 diams.) 

 2. Phyllite (shde 13527b) on the shore south of Eilean Dubh. The first or 

 slaty cleavage is parallel to the folded bedding and quartz-veins, 

 and the strain-slip cleavage, as before, coincides with the axial plane* 

 of the folds. ( X 18 diams.) 



Discussion, 



Dr. Teall said that two movements were strikingly evident in 

 many parts of the Southern Highlands, but he did not think it safe 

 to infer that the two movements in one locality were necessarily 

 contemporaneous with the two movements in another locality. 

 The Author had conclusively shown that in Colonsay the twa 

 dominant movements were separated by an intrusion of igneous, 

 rocks, and this represented an important step in advance. 



Dr. Flett said that the Author had been fortunate in working in 

 a district where the structures produced by the first movement had 

 not been obliterated by the second movement. In the speaker's 

 experience, it was exceptional to find the effects of two periods of 

 pressure so well characterized and so well preserved. The plutonic 

 rocks described from Colonsay by the Author (syenites, diorites, and 

 kentallenites) had the stamp of the newer granite-series of the 

 Scottish Highlands. These intrusions had been proved to be early 

 Devonian, or perhaps in part late Silurian. The age of the first 

 movement was not quite clear, but it might possibly be an earlier 

 stage of the same process of folding. In that case, Colonsay 

 furnished an instance of two epochs of earth-pressure, separated by 

 a period during which pressure was relieved and igneous dykes were 

 injected. Similar phenomena were very well shown in the Lizard 

 district of Cornwall. 



Mr. Lampltjgh said that, as the last speaker had referred to a 

 possible relation between the Colonsay movements and those in the 

 South- West of England, it might be worth recalling that a sequence 

 of movements had likewise been detected in an intermediate area — 

 the Isle of Man — where, again, the dyke-rocks seemed to have been 



