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ME. W. B. TTEIGHT OX THE 



[May 1908, 



puckering of these laminae is observable, but little or no cleavage 

 yet accompanies it. In Zone 3 the puckering becomes acute and 

 the accompanying cleavage well marked, and as Zone 4 is ap- 

 proached its planes become so close as almost completely to destroy 

 any facility of splitting along the first cleavage. 



The relation of the second cleavage to a corresponding system of 

 folds is, as might be expected, far more obvious than in the case 

 of the first cleavage. It preserves, at least in the softer beds, a 

 strict general parallelism with the axial planes of these folds, and 



undoubtedly owes its 

 Eig. 3. — Section about 2 feet long on the coast 

 north of Port na Cuilce (Colonsay), showing 

 a small lamprophyre sill (L) and quartz- 

 veins (Q,) i?i phyUite. 



origin to the same 

 compressive forces 



,?^^|?J;^^^^'' 



which produced them . 

 We have, in the two 

 cleavages of the 

 islands and in the 

 folding accompany- 

 ing them, the record 

 of two periods of 

 stress, and conse- 

 quent movement, of 

 which one is clearly 

 later than the other. 

 How much later re- 

 mains to be deter- 

 mined. 



The analogy with 

 the sequence of events 

 described by Mr. C. 



Clough ^ in the 



[The quartz-veins lie along the planes of the fu'st 



cleavage, and the lamprophjre sill has also been 



intruded along the same set of planes. Both 



the quartz-veins and the lamprophyre have been 



folded and cleaved by the second movement.] 



Cowal district of 

 Argyll is remarkable. In both instances there is a primary ' folia- 

 tion ' of the rocks, and in both the planes of this foliation are 

 affected by folding and cleavage due to secondary movement. 

 Mr. Clough held that these two movements were distinct, because 

 the jDlanes of secondary cleavage cut not only those of the first 

 cleavage, but also the quartz-veins formed along the latter. 

 AVhere actual cleavage has not been developed, the effect of the 

 later movement can still be recognized in the folding and crinkling 

 of both the early cleavage and the accompanying quartz-veins. 

 An exactly similar relation occurs in Colonsay. Quartz-veins clearly 

 formed along the first cleavage have been folded and even cleaved 

 daring the second movement (see fig. 3). Still, good as is this 

 evidence, it is, perhaps, not quite convincing. We have had the 

 good fortune in Colonsay to come upon better. 



1 *The G-eology of Cowal ' Mem. Geol. Surv. Scotl. 1897, chapt. ii. 



