﻿Yol. 64.] 



TWO EARTH-MOVEMENTS OF COLONSAY. 



301 



can be no doubt that it was x-)roduced, during a period of movement, 

 in the planes normal to the greatest pressure. In the northern 

 part of Colonsay it preserves a remarkable parallelism to the 

 bedding, but may be seeu here and there to cross it on the crest of 

 an isoclinal fold, having been produced at the same time as this 

 fold and along its axial plane. In the south of the islands we find 

 what appears to be the same cleavage crossing the bedding over 

 large areas without the accompaniment of any such folding. In its 

 production, the arrangement of the pre-existing clastic micas in 



Fig. 2. — Diagram illustrating hy shading 

 the general increase in the intensity of 

 the second movement from west to east, 

 the curved lines being lines of approxi- 

 mately equal movement. Zone 1 is un- 

 shaded, Zones 2, 3, and 4 are shown 

 ivith increasing degrees of shading. 



planes perpendicular to 

 the greatest stress has 

 probably gone hand in 

 hand with the develop- 

 ment of secondary seri- 

 citic mica along these 

 planes. This growth of 

 secondary minerals in a 

 definite series of planes, 

 however close these 

 ]3lanes may be, is an 

 incipient type of folia- 

 tion. We shall, how- 

 ever, in future refer to it 

 as the first cleavage. 

 On the western side 

 of the islands the first 

 cleavage presents plane 

 or only slightly wavy 

 faces, but eastwards it 

 becomes progressively 

 more folded and crinkled, 

 and on the eastern coast 

 has been subjected to 

 intense puckering. At 

 the same time a new 

 cleavage is developed, 

 which, slight at first, 

 becomes towards the 

 eastern coast fairly well 

 marked, and in the 

 extreme north - east 

 entirely dominates the 

 minor features in the 

 structure of the country. 

 This relation is ex- 

 pressed in fig. 2, in 

 which zones of increasing intensity of both folding and cleavage are 

 shown by difi'erent degrees of shading. In Zone 1 the effect is 

 trivial, amounting only to a slight waviness of the laminae of the 

 first cleavage in the more politic beds. In Zone 2 a pronounced 



