Mr. Hopkins on the Mechanism of Glacial Motion. IV. 333 



Fig. 2. 



or the lines along which the particles moved past each other ; 

 the finer lines those of greatest tangential action, and the dot- 

 ted lines those of maximum tension, or those along which the 

 dra^i takes place towards the middle of the glacier. Conse- 

 quently these latter lines are those along which, according to 

 Prof. Forbes's views, the sliding of one particle past another 

 ought to have taken place. The lines on one longitudinal 

 half only of the mass are represented in the diagram. 



In experiments like those of Prof. Forbes, made with plaster 

 of Paris in a semifluid state, the mass at the extreme upper 

 section of the trough, partly from its adhesion to the trough 

 and partly from its fluid condition, remains stationary, and 

 consequently there is a tendency in the particles along each 

 longitudinal line (more especially along the middle of the 

 trough), to separate from each other. In this respect the mo- 

 tion differs from that in my experiments as above described ; 

 but with regard to the relative motions of the middle and 

 sides, the cases are precisely similar. In the experiments 

 above described with the soft mortar, the lines of dislocation 

 were straight lines ; but when the experiment was varied so 

 as to give an elongation to the central portion (in the manner 

 already described), the lines of dislocation were continued as 

 curved lines across the central portion, forming elongated loops, 

 with their convexity turned in a direction opposite to that in 

 which the motion took place. These elongated loops exactly 

 resembled those indicated by the coloured powder spread on 

 the surface of the plaster of Paris in Prof. Forbes's experi- 

 ments. I may also add, that in repeating the Professor's ex- 

 periments, previously to making the others, I had observed 

 that when discontinuity in the motion became sensible, as it 

 did near the flanks if the inclination of the trough was made 

 sufficiently great, it took place by a sliding of the particles 

 past each other along the curved loop, as in my experiments. 

 This was shown by a line, originally continuous across the 

 loop, becoming a broken line at the point of intersection. The 

 two systems of curves are undoubtedly identical. 



It appears, then, that if a mass move in the same manner 

 as a glacier in a canal-shaped valley (independently of local 

 obstacles), two systems of dislocations may be formed, accord- 



