The Theory of Glacier motion. 109 



case the efficiency of gravity is principally due to the 

 state of disintegration of the lower surface of the glacier, 

 while in the other it is maintained that its efficiency is 

 due to the plasticity of the general mass. He does not 

 deny that ice may be partially plastic, but while admitting 

 this he urges that much the greater portion of the move- 

 ment is due to sliding. He also, with singular fairness, 

 appeals to further experiment as the real test, and he states 

 that the observations required are such as will determine, 

 as far as possible, the relative motions of the upper and 

 lower surfaces of a glacier, and he admits that if experiment 

 proves the motion of the upper part of a glacier to bear a 

 large ratio to that of the lower surface, the claims of the 

 viscous theory must be at once admitted {Phil. Mag.^ 1845. 

 XXVI. 247 — 250). The test last mentioned was again 

 -appealed to by Mr. Hopkins in a later paper, in which he 

 says : "The ultimate test of the sliding and plastic theories 

 must be sought in observations on the relative motions of 

 the upper and lower surfaces of a glacier. The claims of 

 the two theories would thus be decided beyond dispute. 

 Accurate observations are also required to ascertain 

 the form which a continuous straight line drawn on 

 the surface of a glacier and perpendicular to its axis will 

 assume by the more rapid motion of its central portion. 

 Will it be deformed into a continuous loop or into a 

 a discontinuous one. Such observations would decide the 

 degree in which the greater central motion is due to the 

 flexibility or plasticity of glacial ice, and the degree in 

 which it is attributable to the dislocation of the general 

 mass. Observations of both kinds are become essential in 

 the present state of glacial theories, and would do much 

 more towards settling the question at once respecting the 

 cause of glacial motion, than any further controversy." 

 {Phil. Mag., XXVI. 599). The appeal was not declined by 

 Forbes. 



