Mr. J. Ball on the Formation of Alpine Lakes. 491 



of glaciers to excavate deep lake-basins — it will be convenient 

 to fix attention upon a single case. The same arguments will 

 apply, mutatis mutandis, to others. Taking, then, the former 

 glacier of the Tessin, which descended into the basin now occu- 

 pied by the Lago Maggiore, and leaving out of account the 

 branch of the Toce glacier which entered the lake-basin between 

 Pallanza and Baveno, we have the following conditions under 

 which the proposed theory must be tested. Admitting that the 

 lake-basin has been partly filled up in modern times, the ancient 

 glacier of the Tessin, formed by the union of many ice-streams, of 

 which the most important was that flowing from the Val Leventina 

 by Faido, reached the level of the lake-basin at or near Bellinzona. 

 The distance from Faido to that town is about 23 miles, and in 

 that space the bed of the valley falls 1573 feet, so that the average 

 slope is little more than 1 in 80. From Bellinzona to the lower 

 end of the lake an approximately level bed of diluvium would, 

 according to the theory, have extended to the lower end of the 

 lake, a distance of 47 miles. The basin or trough containing 

 this supposed mass of diluvium is in the form of a long valley, 

 for the most part enclosed between steep walls of rock, very 

 sinuous in form, having an average breadth of about two miles, 

 but contracted at one part to about one mile, and enlarged else- 

 where to a breadth of three miles. The depth of the trough 

 throughout a great part of its length considerably exceeds 1500 

 feet, but at one point, about 34 miles from Bellinzona, it exceeds 

 2600 feet. A rough estimate derived from the ascertained 

 depth of the lake gives the probable contents, supposing it filled 

 up to its present level, at from 15 to 20 cubic miles. The 

 reader is requested to consider how a glacier, under the circum- 

 stances here described, could have cleared out this prodigious 

 amount of solid matter. It does not appear to me that those 

 who have written on the subject have considered the mechanical 

 problem at all closely. 



The removal must have been effected, if at all, either by the 

 front or tongue of the glacier while it advanced through the 

 channel, or else by some forces brought into play when the mass 

 of diluvium was covered by that of the glacier. I am willing 

 to admit, as I have done in a former paper, that the tongue of 

 the advancing glacier would probably have some slight effect in 

 removing a superficial layer of such matter as the diluvium; 

 but 1 see no reason to hold with M. Gastaldi that the effect in 

 the case of a gigantic glacier like that in question would be much 

 greater than that seen in existing glaciers. There is no reason 

 why the tongue of a glacier 100 miles long should be thicker 

 than that of one five miles long. The thickness depends on the 

 relation between the rate of advance of the glacier and the rate 



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