150 Mr. H. H. Howorth on 



We may take it, therefore, as clearly proved, that glacier- 

 ice is not a rigid body, but a plastic one ; and that its 

 movements may be compared with those of pitch or other 

 plastic substances, whose several parts can roll over one 

 another. When ice moves under the influence of gravity, 

 except on very rapid slopes, it acts like other plastic 

 substances act. Its lower surface, in contact with the 

 ground, is dragged by friction, and moves very little, 

 while its upper part flows faster. If we pour pitch on a 

 table, we find that it spreads out, not by the bottom of the 

 mass spreading, but by the edges rolling over; the upper 

 stratum curling round to form the lower one, which is 

 dragged by the surface of the table. Just as a drop of 

 water rolls down a plain, leaving in its track the successive 

 bottom layers of itself 



In claiming ice as a plastic substance I do not mean that 

 it is completely plastic, but that it behaves like sealing wax 

 and other similar bodies, which mould themselves with 

 time to the surfaces on which they lie, even at moderate 

 atmospheric pressures, and maintain, meantime, the 

 quality of excessive brittleness under a blow or rapid 

 change of form. The very fact of its cracking and forming 

 crevasses shows that it is not perfectly plastic, but under 

 certain conditions of tension will snap like a brittle sub- 

 stance. Its viscosity doubtless also varies both with its 

 temperature, as Forbes urged, and also with the character 

 of its molecular structure ; and we may conclude as the 

 result of our inquiry, that the motion of a glacier is due 

 in the main to the actual flow of its substance, which 

 goes on continuously, and, secondly, to a certain sliding over 

 its bed, and certain more sudden movements due to large 

 masses cracking asunder under great tension, the first being 

 no doubt much the most potent and influential of these 

 causes. So far as we can judge, none of its motion is due to 

 molecular movements other than those induced by gravity, 



