Prof. Tyndall on the Conformation of the Alps. 257 



as upon the river. The action of water upon some rocks is almost 

 purely mechanical; they are cut away, sometimes in sensible 

 masses. In other cases the action is chemical as well as mecha- 

 nical. Water, in passing over limestone, charges itself with car- 

 bonate of lime without damage to its transparency ; the rock 

 is dissolved in the water; aDd 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 ascribed 

 the fantastic forms which this rock usually assumes, and also the 

 grottos and caverns which interpenetrate limestone formations. 

 A rock capable of being thus dissolved will expose a smooth sur- 

 face after the water has quitted it ; and in the case of the Via Mala 

 it is the polish of the surfaces, and also 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 which is bounded by 

 high terraces, evidently cut by water. 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, immedi- 

 ately informed me that such was the tradition of the neighbour- 

 hood. This man conversed with intelligence, and as I drew his 

 attention to the rolled stones, which rest not only above the river 

 but above the road, and inferred that the river must have been 

 there to have rolled those stones, he saw the force of the evidence 

 perfectly. In fact in former times, and subsequent to the retreat 

 of the great glaciers, a rocky barrier crossed the valley at this 

 place, damming the river which came from the residual glaciers 

 higher up. A lake was thus formed which poured its waters 

 over the barrier. Two actions were here at work, both tending 

 to obliterate the lake — the raising of its bed by the deposition of 

 detritus, and the cutting of its dam by the river. In process of 

 time the cut deepened into the Via Mala ; the lake was drained, 

 and the river now flows in a definite channel through the plain 

 which its waters once totally covered. 



From Tusis I crossed to Tiefcnkasten by the Schien Pass, and 

 thence over the Julier Pass to Pontrcsina. There are three or 

 four ancient lake-beds between Tiefcnkasten and the summit of 

 the Julier. They arc all of the same type — a more or less broad 

 and level valley-bottom, with a barrier in front through which 

 the river has cut a passage, the drainage of the lake being the con- 

 sequence. These lakes were sometimes dammed by barriers of 

 rock, sometimes by the moraines of ancient glaciers. An exam- 

 ple of this latter kind occurs in the llosegg valley, about twenty 

 minutes below the end of the llosegg glacier, and about an hour 

 from Pontresina. The valley here is crossed by a pine-covered 



