54 SIXTH LETTER ON GLACIERS. [1844. 



If, as we cannot doubt, the slower motion of the glacier 

 near its sides be owing to the retardation which their excessive 

 friction occasions, there must necessarily be a retardation at the 

 bottom in a similar manner, and the surface of the glacier will 

 move faster than the strata in contact with the ground ; to 

 which it is even supposable, that, in some cases, they may be 

 entirely frozen. This retardation may, perhaps, be less than 

 the lateral retardation, because the slope of the valley in which 

 the glacier lies is probably more even, generally speaking, than 

 its breadth is regular. In fact, so great is the irregularity of 

 the ground-plan of any compound valley so frequent the inter- 

 fering ridges or promontories, the bays formed by adjoining 

 tributary valleys and so numerous the gorges or contractions 

 that we cannot properly call the lateral resistance to the 

 onward motion of a glacier, friction, but rather a direct opposi- 

 tion to the exit of a solid body, which renders its plasticity 

 absolutely essential to its progression. Nevertheless, the infe- 

 rior slope of the glacier bed being also irregular, and its friction 

 great, must cause a retardation in the lower strata of ice, which 

 must be continually overtaken by the superior ones : and this 

 appears to me to be so plain and necessary a consequence of 

 the combination of facts which we have to consider, that per- 

 haps the direct proof of it would not repay the labour which it 

 would involve, which would be of the most serious kind ; for 

 we must not expect to find the difference of velocity apparent 

 in the superficial strata, even to a considerable depth, since we 

 know that the retardation is a maximum near the sides and 

 bottom, and that, for the same reason, the motion of all the 

 central part of a glacier is nearly uniform, so will the motion 

 of all the part of the ice near the surface be nearly uniform. 



These considerations suggest the explanation of a difficulty, 

 kindly suggested to me by a most competent judge, who ex- 

 pressed himself at the same time persuaded of the truth of the 

 viscous theory of glaciers. " How comes it, that, if the motion 

 of the different parts of a glacier diminishes from the surface to 

 the bottom, the * trou de sonde ' or bore, 140 feet deep, made 



