Prof. Tyndall on the Conformation of the Alps. 265 



rocks are of a soft character, the amount of this finely pulverized 

 matter suspended in the water is very great. The water, for 

 example, of the river which flows from Santa Catarina to Bormio 

 is thick with it. The Rhine is charged with this matter, and by 

 it has so silted up the Lake of Constance as to abolish it for a 

 large fraction of its length. The Rhone is charged with it, and 

 tens of thousands of acres of cultivable land are formed by it 

 above the Lake of Geneva. In the case of every glacier we have 

 two agents at work, — the ice exerting a crushing force on every 

 point of its bed which bears its weight, and either rasping away 

 this point in powder or tearing it bodily from the rock to which 

 it belongs ; while the water which everywhere circulates upon 

 the bed of the glacier continually washes the detritus away and 

 leaves the rock clean for further abrasion. Confining the action 

 of glaciers to the simple rubbing away of the rocks, and allow- 

 ing them sufficient time to act, it is not a matter of opinion, but 

 a physical certainty, that they will scoop out valleys. But the 

 glacier does more than abrade. Rocks are not homogeneous ; 

 they are intersected by joints and places of weakness, which divide 

 them into virtually detached masses. A glacier is undoubtedly 

 competent to root such masses bodily away. Indeed the a priori 

 consideration of the subject proves the competence of a glacier 

 to deepen its bed. Taking the case of a glacier 1000 feet deep 

 (and some of the older ones were probably three times this depth), 

 and allowing 40 feet of ice to an atmosphere, we find that on 

 every square inch of its bed such a glacier presses with a weight 

 of 375 lbs., and on every square yard of its bed with a weight 

 of 486,000 lbs. With a vertical pressure of this amount the 

 glacier is urged down its valley by the pressure from behind. We 

 can hardly, I think, deny to such a tool the power to excavate. 



While writing these remarks, I have refreshed my memory 

 by reference to the paper of Mr. John Ball, published in 

 the 25th volume of the Philosophical Magazine (Feb. 1863). 

 Mr. Ball's great experience of the Alps is sure to render any- 

 thing he writes regarding them interesting. I have read his paper 

 and attended to his suggestions, but I confess I do not see the 

 cogency of his arguments. An inspection of the map of Switzer- 

 land, with reference to the direction of its valleys, suggests to my 

 mind no objection to the theory of erosion. The perusal of the 

 paper has assured me that Mr. Ball has paid attention to the for- 

 mation of ancient lakes. He deems their beds a prominent feature 

 of Alpine valleys ; and he considers the barriers which dammed 

 them up, and which were not removed by the ancient glaciers, as "a 

 formidable difficultyinthewayof Prof. Tynd all's bold hypothesis.'' 

 " Looking at the operation as a whole," writes Mr. Ball, " it is 

 to me quite inconceivable that a glacier should be competent to 



Phil. Mag. S. 4. Vol. 28. No. 189. Oct. 1864. T 



