378 Professors Tyndall and Huxley on the Structure 



be stated, in general terms, to be the conversion by pressure of 

 irregularly-formed surfaces of weak cohesion into parallel planes. 

 To produce lamination in a compact body such as wax, it is mani- 

 fest, that while it yields to the compression in one direction, it 

 must have an opportunity of expanding in a direction at right 

 angles to that in which the pressure is exerted ; a second cause 

 is the lateral sliding of the particles which thus takes place, and 

 which may be very influential in producing the cleavage*. 



Before attempting to show the connexion between this theory 

 and the case at present under consideration, a mode of experi- 

 ment may be described which was found to assist in forming a 

 conception of the mechanical conditions of a glacier, and which 

 has already been resorted to by Professor Forbes in demonstra- 

 tion of the viscous theory. Owing to the property of ice de- 

 scribed in § 3, the resemblance between the motion of a sub- 

 stance like mud and that of a glacier is so great, that consider- 

 able insight regarding the deportment of the latter may be de- 

 rived from a study of the former. From the manner in which 

 mud yields when subjected to mechanical strain, we may infer 

 the manner in which ice would be solicited to yield under the 

 same circumstances. 



To represent then the principal accidents of a glacial valley, a 

 wooden trough, A B C D, fig. 8, of varying width and inclina- 

 tion, was made use of. From A to C the trough measures 6 

 feet, and from A to B, 15 inches. It is divided into five seg- 

 ments ; that between A B and ef is level, or nearly so, that 

 between ef and g h is inclined ; from gh to ik is again nearly 

 level ; from i k to m n inclined, while from mn to CD the incli- 

 nation is less than between i k and m n. The section of the bot- 



* Three principal causes may ojjerate in producing cleavage : — 1st, the 

 reducing of surfaces of weak cohesion to parallel planes; 2nd, the flattening 

 of minute cavities; and 3rd, the weakening of cohesion by tangential action. 

 The third action is exemplified by the state of the rails near a station where 

 the break is applied. In this case, while the weight of the train presses 

 vertically, its motion tends to cause longitudinal sliding of the particles of 

 the rail. Tangential action does not however necessarily imply a force of 

 the latter kind. When a solid cylinder, an inch in height, is squeezed by 

 vertical pressure to a cake a quai'ter of an inch in height, it is impossible, 

 physically speaking, that the particles situated in the same vertical line 

 shall move laterally with the same velocity ; but if they do not, the cohe- 

 sion between them will be weakened or ruptured. The pressure will ])ro- 

 duce new contact, and if the new contact have a cohesive value equal to that 

 of the old, no cleavage from this cause can arise. The relative capacities 

 of different substances for cleavage, appears to depend in a great measure 

 upon their different properties in this respect. In butter, for example, the 

 new attachments are equal, or nearly so, to the old, and the cleavage is 

 consequently indistinct ; in wax this does not appear to be the case, and 

 hence may arise in a great degree the perfection of its cleavage. The fur- 

 ther examination of this subject promises interesting results. 



