176 FRAGMENTS OF SCIENCE. 



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 favor of 

 the cutting power of the river. For if floods operating tit 

 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 upward, and from a point near another 

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

 portion 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 unmistakable marks of erosion. This 

 cleft was best seen on looking downward from a point 

 near the bridge; but looking upward from the bridge 

 itself, the evidence of aqueous erosion was equally con- 

 vincing. 



The character of the erosion depends upon the rock as 

 well as upon the river. The action of water upon some 

 rocks is almost purely mechanical; they are simply ground 

 away or detached in sensible masses. Water, however, in 

 passing over limestone, charges itself with carbonate of 

 lime without damage to its transparency; the rock is dis- 

 solved in the water; and the gorges cut by water in such 

 rocks often resemble those cut in the ice of glaciers by glacier 

 streams. To the solubility of limestone is probably to be as- 

 cribed the fantastic forms which peaks of this rock usually 

 assume and alsd the grottos and caverns which interpenetrate 

 limestone formations. A rock capable of being thus dissolved 

 will expose a smooth surface after the water has quitted it; 

 and in the case of the Via Mala it is the polish of the sur- 

 faces and the curved hollows scooped in the sides of the 

 gorge, which assure us that the chasm has been the work 

 of the river. 



About four miles f*om Tusis, and not far from the little 

 village of Zillis, the Via Mala opens into a plain bounded 

 by high terraces. It occurred to me the moment I saw it 

 that the plain had been the bed of an ancient lake; and a 

 farmer, who was my temporary companion, immediately 



