90 Mr. J. Ball on the Formation of 



see that the movement of the lower surface of the ice would be 

 completely stopped, and that the onward movement would be 

 effected by the flowing- of the upper over the lower portions of 

 the mass. It is forgotten that the resistance offered by friction 

 would be increased to an almost infinite extent, while the resist- 

 ance of the substance of the glacier to internal rearrangement 

 is confined within moderate limits. M. Mortillet has estimated 

 that the pressure of the ice upon the bed of certain ancient 

 glaciers occupying lake-basins must have reached 300 and even 

 500 tons per square metre; but if this pressure were even ten 

 times as great, the effect would only more certainly have been 

 to force the ice of the lower surface into every inequality of the 

 bed, and to make the resistance to onward movement more and 

 more insuperable. When the front of the glacier is supposed to 

 move up a slope, as in Mr. Ramsay's diagram of the Lake of 

 Geneva, the above reasoning applies a fortiori. If, on further 

 consideration, Professor Ramsay should feel doubtful on this 

 point, I would urge him to make direct observations, which may 

 tend to satisfy his own mind, and that of others who may natu- 

 rally be swayed by his authority. It is not easy to find the 

 conditions under which such observations could most usefully 

 be attempted; but the glacier of La Brenva, and some others 

 where the ice impinges against fixed obstacles, may furnish matter 

 of instruction. 



It will be obvious to the reader, that the question here dis- 

 cussed has a considerable bearing upon Professor TyndalPs 

 views of the formation of valleys. The reason why the action 

 of a glacier is more limited than appears probable to one who 

 considers the enormous mechanical power developed in its pro- 

 gress, is because the greater part of that power is expended in 

 overcoming the resistance of the mass to internal rearrangement, 

 and a small portion only is applied to the onward motion of its 

 lower surface, by which alone any mechanical effect is produced 

 on the subjacent rocks. 



If it should not appear superfluous to discuss at greater 

 length the mechanical question involved in Mr. Ramsay's theory, 

 I might ask how we are to believe that the glacier of the Rhone, 

 after it had flowed out of its native valley, when, being no 

 longer confined within a defined channel, it had covered with 

 ice the plain of Switzerland as far as the Jura, was yet able 

 to excavate, a rock basin 50 miles and more in length, 12 

 miles broad, and nearly 1000 feet deep, while at the point 

 where its mechanical power was at its maximum, in the defile 

 of St. Maurice, it has failed to cut through rocks of no extreme 

 hardness a more spacious opening than that which the traveller 

 still sees between Bex and Martigny. It is a complete under- 



