﻿306 3IE. AY. B. WEIGHT OX THE [^a}' I908, 



We found altogether only two or three pieces of rock which showed 

 the primary cleavage affected by secondary crinkling, and these 

 occurred under such relations that it was probable that the crinkling 

 was impressed on them while in the breccia. They all lay trans- 

 verse to the direction of the secondary cleavage, and w^e found no 

 crinkled fragments in which the primary cleavage laj' parallel to 

 this direction. 



It is important to note that the lamprophyre dykes which 

 traverse the syenite are chilled against it. The syenite must clearly 

 have been cool before their intrusion. 



We may therefore draw the conclusion that, between the first 

 and second movements in the schists, there elapsed sufficient 

 time for — 



(1) the intrusion of a fair-sized plutonic mass ; 



(2) the cooHng of the same ; and 



(3) the intrusion of an extensive series of lamprophyre dykes. 



The other plutonic masses of the island are all later than the first 

 movement, but their relation to the second is not quite so obvious 

 as in the case of the Kiloran syenite. The diorite of Scalasaig is 

 traversed by a lamprophyre dyke showing distinct cleavage, and 

 is therefore presumably of inter-movement age. An attempt to 

 determine microscopically the time-relation of the crinkling to the 

 hornfelsing of this mass and of the Balnahard kentallenite, has 

 not, up to the present, been crowned with success. The occurrence 

 of similar types of the two last-mentioned masses lends support to 

 the idea that they belong to the same suite of intrusions. 



YI. The Varied Age oe the Lampeophyees. 



A very large proportion of the lamprophyres were intruded in 

 the period between the two movements. Many of them were, as 

 we have seen, later than the syenite ; some of them were certainly 

 earlier, for blocks of decomposed lamprophyre have been obtained 

 from the breccia. 



There occur also in Colonsay a certain number of vogesite dykes 

 and sills, which are clearly later than the second movement. These 

 have generally, but not exclusively, an east and west trend, and are 

 in their manner of intrusion much more independent of the structure 

 of the rocks than their predecessors. Two such dykes cross the 

 northern end of the island in an approximately east and west 

 direction, dipping southwards at a considerable angle. On the coast, 

 about a quarter of a mile north of Port na Cuilce, the relation of one 

 of these to the secondary folding is well seen (fig. 8, p. 307). It cuts 

 across and truncates the folds of the secondary system in such a 

 manner as to leave no doubt that it is subsequent to them. East of 

 Port na Cuilce the other is very well exposed, having a thickness of 

 20 to 40 feet. It is very heterogeneous in character, and is charged 



