256 Prof. Tyndall on the Conformation of the Alps. 



rocks by rain and frost), as affecting the forms of the more 

 exposed and elevated peaks. 



I had heard it stated that the Via Mala was a striking illus- 

 tration of the fissure theory — that the profound chasm thus 

 named, and through which the H inter- Ilhem now flows, could 

 be nothing else than a crack in the earth's crust. To the Via 

 Mala this year I therefore went, to instruct myself by actual 

 observation upon the point in question. The gorge commences 

 about a quarter of an hour above the town of Tusis ; and, on 

 entering it, the conclusion which first gains credence is that it 

 must be a fissure. This conclusion in my case was modified as 

 I advanced. Some distance up the gorge I found upon the slopes 

 to my right quantities of rolled stones, evidently rounded by 

 water-action. Still further up, and just before reaching the first 

 bridge which spans the chasm, I found more rolled stones asso- 

 ciated with sand and gravel. Through this mass of detritus, for- 

 tunately, a vertical cutting had been made, which exhibited a sec- 

 tion showing perfect stratification. There was no agency in the 

 place to roll these stones, and to deposit these alternating layers 

 of sand and pebbles, but the river which now rushes some hun- 

 dreds of feet below them. At one period of the Via Malays history 

 the river must have run at this high level. Other evidences of 

 water-action soon revealed themselves. From the parapet of the 

 first bridge I could see the solid rock 200 feet above the bed of 

 the river scooped and eroded. It is stated in the guide books, 

 that the river, which usually runs along the bottom of the 

 gorge, has been known almost to fill it during violent thunder- 

 storms ; and it may be urged that the marks of erosion which 

 the sides of the chasm exhibit are due to those occasional floods. 

 In reply to this, it may be stated that even the existence of such 

 floods is not well authenticated, and that, if the. supposition were 

 true, it would be an additional argument in favour of the cutting- 

 power of the river. For if floods operating at rare intervals 

 could thus erode the rock, the same agency, acting without 

 ceasing upon the river's bed, must certainly be competent to 

 excavate it. I proceeded upwards, and from a point near another 

 bridge (which of them, I did not note) had a fine view of a por- 

 tion of the gorge. The river here runs at the bottom of a cleft 

 of profound depth, but so narrow that it might be leaped across. 

 That this cleft must be a crack is the impression first produced ; 

 but a brief inspection suffices to prove that it has been cut by 

 the river. From top to bottom we have the unmistakeable marks 

 of erosion. This cleft was best seen by looking downwards from 

 a point near the bridge ; looking upwards from the bridge itself, 

 the evidence of aqueous erosion was equally convincing. 



The character of the erosion depends upon the rock as well 



