176 FBA&MSNTS 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 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 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 lias 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 us 
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 also 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 from 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 
