41-i ON THE CAUSE OF GLACIER MOTION. 



entire course; whereas, as all glaciers originate above the line of per- 

 petual snow, it is obvious that however well this theory may apply to 

 as much of it as the rays of the sun might affect so as to melt it, it is 

 absolutely certain that it could not apply to that portion where the ice 

 does not melt. Another reason must therefore be sought for, that will 

 apply to the glacier as a whole. I venture, then, with all deference, to 

 submit the following as one that, whether it solve all the difficulties of 

 this difficult question or not, is, I think, worth a moment's considera- 

 tion. The alternate thaiowg and freezing, that is, the expansion and 

 contraction of the glacier, of the enormous mass of ice itself, consti- 

 tutes the motive poicer of this extraordinary phenomena. The melting 

 and congealing of the water in the chinks are opposing, not assisting, 

 forces; because, although water in the act of freezing does expand, this 

 takes place geaerally and in the greatest degree at night; therefore the 

 contraction of the bulk of the water is not coincident with, but in 

 opposition to the expansion of the volume of the glacier by the action 

 of the sun, which takes place during the day ; and the act of contract- 

 ing, consequent on the diminished temperature, and therefore increased 

 density, or, which is the same"thing, diminished bulk, takes place at 

 night, the very time that, according to Agassiz's theory, the greatest 

 dilatation of the mass ought to take place. These are antagonistic 

 forces, whose effects must be neutralized. These, added to the stupen- 

 dous vis inertiee of the glacier itself, show the amazing power of the 

 apparently simple action of difference of temperature upon inert matter. 

 The resultant is easily predicated. The mass, being once set in motion, 

 moves in accordance with a known universal law of nature, i.e., that 

 expansive forces move in the direction of the least resistance, that is, 

 downwards towards the lower and wider extremity of the valley. It 

 may be, and generally is, assisted by the formation of the sides and bed 

 of the valley and laws of gravitation; though were these aids absent, 

 it would still advance in that direction, because, being frozen and solid 

 at the upper extremity, it could not move towards that. It is therefore 

 shut up to go the other way, that is, in the line of the least resistance. 



This seems to me to be the rationale of the onward motion of modern 

 glaciers ; and I see nothing in it which will bar its application to a 

 system or series of glaciers, however extensive, however great, the 

 operation of the laws which regulate the movements of matter being 

 absolute and invariable. 



Lindsay, February 17, IS 70. 



