LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



SEPTEMBER 1870. 



XIX. On the Cause of the Motion of Glaciers. 

 By James Croll, of the Geological Survey of Scotland*. 



The generally accepted theory proved by the Rev. Canon Moseley 

 to be incorrect. 



SINCE the time that Professor Tyndall had shown that all the 

 phenomena formerly attributed by Professor Forbes to plas- 

 ticity could be explained upon the principle of regelation, disco- 

 vered by Faraday, the viscous theory of glacier-motion has been 

 pretty generally given up. The ice of a glacier is now almost 

 universally believed to be, not a soft plastic substance, but a sub- 

 stance hard, brittle, and unyielding. The power that the glacier 

 has of accommodating itself to the inequalities of its bed without 

 losing its apparent continuity is referred to the property of rege- 

 lation possessed by ice. All this is now plain ; but what is it 

 that impels the glacier forward is still a question under discus- 

 sion. Various theories have been propounded regarding the 

 cause of the descent of glaciers, all of which have been abandoned 

 with the exception of that which attributes their descent to gra- 

 vitation. But as the ice of the glacier descends with a differen- 

 tial motion, we have not only to explain what causes the glacier 

 to slide on its bed, but also what displaces the particles of the ice 

 over one another and alongside one another. What, then, is the 

 force which shears the ice ? The answer generally given is that 

 gravitation alone is the force which does this ; or, in other words, 

 the mere weight of the ice is sufficient to overcome its cohesive 

 force and to displace the particles over one another. The 



* Communicated by the Author. 

 Phil Mag. S. 4. Vol. 40. No. 266. Sept 1870. M 



