﻿308 ME. TV. E. WEIGHT OX THE [^^J I908, 



In the present case we are dealing with what may be called 

 * alternating strain-slip/ by which it is meant to imply that the 

 shearing motion along the various planes of gliding has not been 

 everywhere in the same direction. On two adjacent gliding-planes 

 the motions may be equal and opposite, and thus compensate one 

 another ; or on several successive planes the shear may be con- 

 tinuously in the same direction, and then continuous^ back again 

 on the next succeeding ones. This structure is illustrated in figs. 

 1 & 2, PI. XXXIY. 



It is exactly analogous to cases figured by Prof. Heim ^ and Mr. 

 Harker.^ Owing to the compensating action of the alternation, there 

 may be on a large scale no difierential movement in the direction of the 

 strain-slip cleavage plane. This is one step towards the explanation 

 of the production of the latter in the normal plane. There is a 

 difficult}^, pointed out by Mr. Harker, in supposing any gliding or 

 faulting motion whatsoever strictly perpendicular to the direction 

 of greatest pressure, since there is no component of shear in this 

 direction. Such motion would certainly seem to be theoretically 

 impossible in a rock made up of laminae of absolutely uniform 

 strength. The phyllites in which this structure is produced in 

 Colonsay are, however, far from uniform. In the cases shown in 

 PL XXXIY strong quartzose bands traverse the phyllites parallel 

 to the first or slaty cleavage, and are buckled into folds of which the 

 axial planes are parallel to the second cleavage. "We may presume 

 that each lamina will give rise under compression to folds whose 

 amplitudes are a direct function of its strength. As the movement 

 progresses, the considerable displacement of the crest and trough of 

 the folds in the stronger laminae will draw and thrust to one side 

 the adjacent phyllite, and adjustment will take place most easily by 

 shearing along the limbs of the much smaller folds produced in the 

 mica-laminae of the phyllite. 



The folding and subsequent strain-slip would seem therefore to 

 be the result of stresses acting on a decidedly heterogeneous rock, 

 in which the weaker laminae became folded on a smaller scale, and 

 then adjusted themselves by fold-faults to the form of the stronger 

 laminae. Prof. Heim long ago showed how this structure arose in 

 pelitic bands intercalated between stronger beds of dolomite or grit, 

 the successive strain-slip bands being moved over one another along 

 planes parallel to the axial planes of folding in such a manner as to 

 adjust themselves to the form of the stronger beds : — 



' Die Mikrofaltenrerwerfungen sind jedoch nicht an der umgebogenen Stelle 

 am starksten ausgebildet, sie erlangeii ihren Hohepunkt an beiden Schenkebi 

 zu der Seite der Umbieguugsstelle, wahrend an der letzteren mehr nur wellen- 

 formiger Scbichtenverlauf vorkommt. Zu beiden Seiten gegen die Scbenkel 

 hin beherrschen die VerwerfungsHnien das Bild. Die Eeste der Scbichtung 



^ Op. cit. Atlas, pi. XV. 



^ ' Eeport on Slaty Cleavage & Allied Eock-Structures ' Eep. Brit. Assoc. 

 (Aberdeen) 1885, pp. 838, 840. 



