﻿Vol. 64.] TWO EAETH-MOVEME?^TS OP COLOIS^SAY. 309 



zwischen denselben sind nur bei starker Vergrosserung noch ganz sicher in 

 ihrem S-formig gekrlimmteu Vei'lauf zu erkenneu, iind liegen symmetriscli auf 

 beiden Seiten der Mittellinie der ganzen Uinbiegung. Die Verweri'ungslinien 

 zeigen nach beiden Scbenkeln bin die Tendeuz, dieser Mittellinie parallel sich 

 zu stellen, und scbneiden deshalb die Biegungsscbenkel unter scbiefem Winkel, 

 sie lassen sich mit scliarfer Loupe auf friscbem Brucbe als Rutscbspiegel 

 erkennen. Diese Faltenverwerfungslinien sind Flacben starker Differential- 

 bewegungen, welchen entlang durch Yerscbiebungen das plasticbe Material von 

 den Mittelscbenkeln, wo es gequetscbt wurde, gegen die Umbiegungsstellen 

 a\iswich, und dort hinstromend sich anbaufte.' ^ 



It is worth remarking that similar strain-slip can be observed in 

 connexion with the slaty cleavage of the first movement, the gliding- 

 planes being coincident with the cleavage. Mr. Clough ^ has figured 

 a case of this associated with the analogous slaty cleavage of Cowal. 



An interesting result was obtained by mapping the planes of 

 secondary cleavage, or, where these Avere not well enough developed 

 to be readily observable, the axial planes of secondary folding. The 

 result is shown in fig. 9 (p. 310). The curved lines indicate the strike 

 of the planes, and their dip is shown by the arrows. The constancy 

 of the strike and dip in some areas contrasts with their irregularity 

 in others, where the condition of affairs cannot be adequately 

 expressed on a small scale map. The remarkable variation in the 

 direction of the stresses still awaits explanation. 



In suitable situations in various parts of Colonsay a third system 

 of crinkling is developed, with axes having a general east and west 

 trend. This has the appearance of being later than the second 

 movement just described, but it is difficult to produce any definite 

 evidence on this point. The puckerings are usually so small and feeble 

 that they are difficult to trace, except under exceedingly favourable 

 circumstances, and next to impossible to pick out from the folds of 

 the second movement, except where they cross these latter at a 

 considerable augle. Planes of easy fracture resulting from sporadic 

 zigzag displacement, the ' knickungsebenen ' or buckling-planes of 

 Brogger,^ are found here and there, having the general direction of 

 the planes of the third movement. 



With the exception of a comparison with the district of Cowal, 

 no discussion of analogous results of movement in other districts 

 has been attempted. Thus, for example, no reference is made to 

 the work of Mr. Lamplugh in the Isle of Man, where the rocks have 

 been affected by successive movements separated by periods of 

 intrusion. It is felt that such discussion would be futile, unless 

 grounds for exact correlation could be brought forward. 



^ 'Mecbanismus der Gebirgsbildung ' vol. ii (1878) pp. 52-53. 



2 < The Geology of Cowal' Mem. Geol. Surv. Scotl. 1897, p. 12 (fig. 3). 



3 W. C. Brogger, ' Die Siluriscben Etagen 2 & 3 im Kristianiagebiet & auf 

 Eker ' Kristiania, 1882. Also figured and described by Mr. Harker, Rep. 

 Brit. Assoc. (Aberdeen) 1885, p. 839. 



