.THE ICE AGE AND ITS WORK. 291 



continually changing- outlines of tlie valley bottom. Hence, where 

 great inequalities occur portions of the rocky floor might be bridged 

 over for a considerable s[)ace, and wliere a valley had a narrow 

 V-shaped bottom the sub glacial stream might eat away so much of 

 the ice that the glacier juight rest wholly on the lateral slopes, and 

 hardly touch the bottom at all. On a tolerably wide and level valley 

 bottom, however, the ice would press with its fullest intensity, and 

 its armature of densely jiacked stones and rock frngments would 

 groove and grind the rocky floor over every foot of its surface, and 

 with a rate of motion perhaps greater than that of the existing Green- 

 land and Alaskan glaciers, owiug to the more southern latitude and 

 therefore higher mean temperature of the soil and the ice. At the same 

 time sub-glacial streams, forced onward under great hydrostatic pres- 

 sure, would insinuate themselves into every vacant groove and furrow 

 as each graving tool successively passed on and the one behind it took 

 a slightly difterent position; and thus the glacial mud, the product of 

 the erosion, would be continually washed away, finally escaping at the 

 lower extremity of the glacier, or in some cases getting embayed in 

 rocky hollows where it might remain permanently as masses of clayey 

 "till," packed with stones and compressed by the weight of the ice to 

 the hardness of rock itself. The continual lubrication of the whole 

 valley floor by water forced onward under pressure, together with the 

 ever changing form of the under surface of the glacier as it slowly 

 molded itself to the varying contours of the rocks beneath, would 

 greatly facilitate the onward motion. Owing to these changes of form 

 and the great upward i)ressure of the water iu all the hollows to which 

 it gained access, it seems j)robable that at any one time not more than 

 half the entire bottom surface of the glacier would be in actual contact 

 with the rock, thus greatly reducing the friction; while, as the process 

 of erosion went on, the rock surfaces would become continually smoother 

 and the inequalities less pronounced, so that even when a rock basin 

 had been ground out to a considerable depth the onward motion might 

 be almost as great as at the beginning of the process. - - - 



3. Ohjeetions of modern writers considered. — Prof Bonney and many 

 other writers ask why lakes are so few, though all the chief valleys of 

 the Alps were tilled with ice; and why, for instance, there is no great 

 lake in the Dora Baltea Valley whose glacier produced the great moraines 

 of Ivrea opposite its outlet into the plains of Italy, and which form 

 a chain of hills 15 miles long and l,r)00 feet high? The answer, in the 

 case of the Dora Baltea, is not diflicult, since it almost certainly has 

 had a series of lake basins at Aosta, Verrex, and other places where 

 the broad level valley is now filled with alluvial gravel. But the more 

 important point is the extreme narrowness of the lower part of the val- 

 ley above Donnas and again near its entrance into the valley of the 

 Po. The effect of this would be that the great glacier, ])robably 2,000 

 feet thick or more, would move rapidly in its upper layers, carrying- out 



