﻿Vol. 64.] 



TWO EAETH-MOVEMENTS OF COLONSAT. 



307 



with numerous fragments of vein-quarfcz. A few pieces of phyllite, 

 showing the crinkling jjroduced in the cleavage by the secondary 

 movement, can be seen enclosed in the dyke here and there. The 



crinkling could not in this 



rig. 8. — Marc/in of cm east and west case have been produced 



vogesite cly'lce north of Port na Cuilce after the fragment had 



truncating the folds produced hy the been incorporated in the 



second movement in the first cleavage, dyke, for the numerous 



structural lines close by 

 in the dyke show no such 

 puckering. Petrographi- 

 cally this dyke is a voge- 

 site, but it contains in the 

 eastern part of its course 

 considerably more biotite 

 than in the western. 

 An east and west dyke 

 probably of this age can be seen in Port an Obain, cutting a slightly 

 cleaved dyke having a north and south trend parallel to the primary 

 cleavage. The later of these two dykes is a vogesite ; the earlier, 

 like all the other cleaved lamprophyres, is far too decomposed for 

 identification. 



A 7-foot vogesite-dyke of irregular east and west trend, occurring 

 on the shore west of Kilchattan, is, judging from its remarkable 

 freshness, probably of this age ; and the 15-foot vogesite sill, which 

 runs in a north-easterly and northerly direction across the hills 

 north of Kilchattan, and dips in a westerly direction contrary to the 

 cleavage and bedding, is, with some others iu different parts of the 

 island, to be referred to this period. 



[Scale = about one third of the natural size.] 



YII. The Natuee of the Second Cleavage. 



The secondary cleavage in Colonsay is always, at least in the softer 

 beds, strictly parallel to the axial plane in the corresponding folds, 

 and has therefore been undoubtedly produced in the plane normal 

 to the direction of greatest compression. Emphasis is laid on this 

 very obvious fact, because the cleavage is not a slaty cleavage, but 

 a true strain-slip cleavage. The investigations of many eminent 

 geologists, from Sharpe and Haughton down to Heim, Harker, and 

 Yan Hise, have proved beyond a doubt that slaty cleavage has in 

 all cases been ]3roduced in the normal plane. No such confident 

 general assertion can be made with regard to strain-slip cleavage, 

 which, on account of its analogy to thrusting, has been supposed by 

 some to have probably been produced parallel to the planes of 

 easiest shearing. Indeed, it is not at all improbable that structures 

 arising in diff'erent ways may be at present grouped under the term 

 'strain-slip,' which is generally understood to be equivalent to the 

 ' ausweichungsclivage ' of Heim.^ 



^ ' Mechanismus der Gebirgsbildung ' Basel, 1878, vol. ii, p. 54. 



