94 Mr. H. H. Ho worth on 



which to advance ; for in passing from the solid to the liquid 

 state its volume is diminished by about one-tenth, and it con- 

 sequently can descend. But when it again assumes the solid 

 form, it will regain its former volume ; but the question is 

 will it go back to its old position. If there were only this 

 one molecule affected by the heat, this molecule would 

 certainly not descend ; but all the molecules are similarly 

 affected, although not all at the same moment of time. At 

 the lower end of a glacier a molecule receives heat from the 

 sun, melts, losing its shearing force, descends by its own 

 weight, and contracts. The next molecule above it is then 

 at liberty to descend, and will do so as soon as it assumes 

 the liquid state. The former molecule has meanwhile 

 become solid, and again fixed by shearing force, but it is 

 not fixed in the old position, but a little below where it was 

 before. If the second molecule has not meanwhile melted 

 from heat derived from the sun, the additional supply it will 

 receive from the solidifying of the first one will melt it. 

 It will then immediately descend till it reaches the first 

 molecule, when it in turn becomes solidified, and so the 

 process goes through the glacier, which is consequently in 

 a state of constant motion downwards " {Phil. Trans.y 



XXXVII. 20I — 204). 



To this exposition of Mr. Croll's theory, Mr. J. Burns 

 replied in the Geological Magazine for 1876: "It seems 

 strange," he says, "that a molecule A should on freezing give 

 its heat to B, from which it is some distance removed, and 

 should impart none to the molecule on which it rests, or to 

 those on either side, but supposing the molecules to be 

 perfectly accommodating in this respect, their downward 

 progress is only helped by heat passing along the glacier 

 from its lower end upwards. Let us take the molecules 

 A, B, and C, somewhere within or on the surface of the 

 glacier. Supposing B now is melted by the sun. When a 

 solid, it was in contact with both A and C, and therefore on. 



