314 GEOLOGY. 



heads of glaciers where the temperature is lowest and the material 

 most loosely granular. In this condition, there is reason to believe 

 that motion takes place between the grains, rather than by their dis- 

 tortion through the displacement of their laminae. The fact that the 

 granular structure is not destroyed, as it would be by the indefinite 

 sliding of the crystal plates over each other, sustains this view. The 

 inference is that the gliding planes play a notable role in glacial move- 

 ment only in the basal parts of the lower ends of glaciers, where the great- 

 est thrusts are developed, and where the granules have become largest 

 and most completely interlocked. At the heads of glaciers, where 

 motion is initiated, there may be great downward pressure, but not 

 vigorous thrusts from behind, and probably only moderate thrusts 

 developed within the body itself. There seems therefore no escape 

 from the conclusion that the primal cause of glacial motion is one which 

 may operate even under the relatively low temperatures, the relatively 

 dry conditions, and the relatively granular textures which affect the 

 heads of glaciers. These considerations lead to the view that move- 

 ment takes place by the minute individual movements of the grains 

 upon one another. While they are in the spheroidal form, as in the 

 neve, this would not seem to be at all difficult. They may rotate and 

 slide over each other as the weight of the snow increases ; but as they 

 become interlocked by growth, both rotation and sliding must appar- 

 ently encounter more resistance. The amount of rotary motion required 

 of an individual granule is, however, surprisingly small, and the melt- 

 ings and refreezings incident to shifling pressures and tensions, and to 

 the growth of the granules, seem adequate to meet the requirements. 

 In order to account for a movement of three feet per day in a glacier 

 six miles long, the mean motion of the average granule relative to its 

 neighbor would be, roundly, yo^o o of its own diameter per day, or 

 one diameter in 10,000 days; in other words, it would change its re- 

 lations to its neighbors to the extent of its diameter in about thirty 

 years. A change of so great slowness under the conditions of 

 granular alteration can scarcely be thought incredible, or even im- 

 probable, in spite of the interlocking which the granules may develop. 

 The movement is supposed to be permitted chiefly by the temporary 

 passage of minute portions of the granules into the fluid form at the 

 points of greatest compression, the transfer of the moisture to adjoin- 

 ing points, and its resolidification. The points of greatest compression 



