66 



Mr. HOPKINS, ON THE MOTION OF GLACIERS. 



these directions, and would therefore be curved with their convexity towards the higher extre- 

 mity of the glacier, the glacial valley being convergent. The degree of curvature would de- 

 pend on the convergency of the lines of motion. If the mass be more or less dislocated, there 

 will still be a prevailing tendency to cause fissures to open in the same direction, though their 

 formation will necessarily be modified by the pre-existing dislocation. There will be the greatest 

 tendency to form these transverse fissures, or crevasses, where the change of velocity is most 

 rapid, or where lateral or other obstacles produce the greatest irregularity of motion. This 

 accounts for the permanent existence of systems of crevasses in particular localities, as already 

 noticed (Art. 7). Particular local causes may produce tensions which are not longitudinal, and, 

 therefore, crevasses which deviate from the general law of formation ; but the general transversal 

 directions of these fissures proves beyond doubt the predominance of a general longitudinal 

 tension during the period of their formation. 



This period, according to our theory, would be the summer, as already shewn (Art. 11). 

 In the winter, it has been also shewn, the motion must probably tend to produce in general an 

 internal longitudinal pressure, and therefore to close previously existing fissures. And here it 

 should be remarked, that it is not essential in order to produce these latter effects, that the 

 motion of the glacier near its upper extremity should be absolutely greater than that near its 

 lower end, but that the former of these motions should bear a greater proportion to the latter 

 during winter, than during summer. 



19. In our previous reasoning the glacial valley has been sup- 

 posed to be convergent in descending. Let us now suppose its width 

 to increase, and its sides to become divergent below CC It is a very 

 general law in such valleys, that where the valley expands its descent 

 becomes less rapid. Assuming such to be the case, the part of the 

 glacier below CC will tend to move more slowly than the part above 

 that line. Consequently, the former of these portions will be in a state 

 of longitudinal compression, which will prevent the general forma- 

 tion of transverse fissures. Also the pressure along CC', which will 

 be greatest in the centre, will push forward the mass below, so as 

 to make it tend to move along diverging lines of motion. Hence if the 

 mass remain continuous it will be in a state of transversal tension, 

 or if the continuity be broken, a system of longitudinal diverging cre- 

 vasses will be formed. Such systems have been recognized both by 

 M. Agassiz and Professor Forbes. 



20. Passage of a Glacier through a narrow Strait Let us suppose the glacial valley to 



contract suddenly at BB' in the following diagram, and consider the motion of the glacier after 

 it arrives at that section. Conceive the mass divided into different portions by longitudinal planes 

 of discontinuity, as in the figure. The central portion cdd'c represents, as before, that in which 

 no sliding of one part of it past another takes place, the planes where this relative motion begins 

 being cd and c'd'. The more the central motion is impeded the greater will be the force/,, (Art. 14), 

 and the narrower will be the breadth dd'. The motion of the lateral portions will be much impeded 

 in such a case as that represented, and near to B and B' may be entirely arrested, but there will be 

 no action which can destroy the motion of the part cdd'c. The central portion, bounded by the 

 planes of discontinuity through B and B', will in fact move very much in the same manner as if 

 those planes were the immoveable boundaries of the glacial valley. 



Unless BB' be too narrow, therefore, the motion of the glacier will be only retarded and not 

 destroyed ; but even this retardation may be counteracted by other causes. The eflfectivencss of 

 the subglacial currents will be increased by the contraction of the valley, and very generally the 

 inclination of a valley increases as its width diminishes. These causes may compensate for the 



