152 Proceedings of the Boyal Physical Society. 



And it is not difficult to see that in the case of a thick mass 

 of ice whose upper surface is exposed to great cold, that the 

 parts of such a mass of ice in contact with the rock will move 

 past the rock at a rate which may well be equal to the 

 average rate of flow at the surface. As this movement takes 

 place under great pressure (each thousand feet of thickness 

 of ice pressing on its rocky bed with a pressure equal to 

 25J tons), the amount of erosion accomplished by a stone- 

 shod mass such as this must be enormous. Little wonder that 

 deep and wide grooves were gouged out of hard rock by the 

 passage of land-ice over it, and little wonder, also, that old 

 river- valleys were deepened into fiords, or were widened and 

 deepened into lake-basins by the prolonged action of this 

 same cause. 



The effect of solar radiation upon stones that have become 

 enveloped in ice is to co-operate with the onward movement, 

 with "ablation and turgescence," and with the frontal re- 

 sistance of the distal parts of the ice, in elevating the stones 

 to the surface along a curve rising from the bottom of the 

 ice upward and outward. In the case of streams of ice such 

 as occurred here during the Glacial Period, the outward path 

 of the included boulders was liable to many deflecting 

 causes ; but, on the whole, it may be said to have moved in 

 the direction of least resistance, which may have pointed to 

 a place very many miles away, where there was some local 

 relief of pressure, brought about, perhaps, by melting, by the 

 calving of icebergs, or by other causes. 



The same cause which, though many miles away, influenced 

 the course a boulder would take under ordinary conditions, 

 came into operation in the cases where a glacial or other 

 barrier was thrown across its path by an ice-stream. There 

 are many instances of this kind, in North Britain especially, 

 as, for example, that of the Galloway ice, which encountered 

 the ice from the English Lake District upon the low ground 

 of the Solway. The contest between the Scottish ice and the 

 English, in this case, was a severe one, and ended in that 

 from Galloway overmastering its English opponent, or, at 

 any rate, in very materially influencing the course the con- 

 joined streams were compelled to take. Another still more 



