Mode of Action of Sir covis on Granite, Porphyry, ^c. 11 



lime scale in the Allee Blanche ; and in two of the lateral glens 

 of the Valorsine, the torrents of La Poyaz and Barbarine flow at 

 the junction of the granite and the slate. Another appearance, 

 also dependent upon structure, and upon the subordination of 

 aqueous action to that structure, is seen in the parallelism and 

 conformity of valleys excavated in the same mountain-ridge. If 

 we look along a straight valley situated at the base of such a 

 ridge, the summits of the minor ridges separating the lateral 

 valleys, which descend iilto the principal valley, form so many 

 parallel lines. The sketch (Fig. 7.) is an exact outline of seve- 

 ral of the ridges of Mont Blanc, as seen from the eminence of 

 La Flegere, and shows the similarity of the valleys which con- 

 tain the successive glaciers of Les Pelerins, Bossons, Taco- 

 naz, &c. 



On the harder and less separable rocks, such as limestone and 

 slate in thick solid strata, or granite and porphyry, the action 

 of streams is far more gradual, and is accomplished by a very 

 distinct and curious process. The stones, whirled round by the 

 water, form hollow cylinders at the bottom, and segments of 

 such cylinders in the sides of the channel ; and these cylindrical 

 impressions go on multiplying, deepening, and enlarging, until 

 they intersect one another, or the seams and cleavages of the 

 rock, and thus separate it into fragments of all shapes and sizes. 

 The bed of the Avon, in Lanarkshire, a short way from its 

 junction with the Clyde, is thickly perforated with holes, about 

 the size and shape of a drum of figs. Traces of the same ope- 

 ration may be observed in the Clyde itself about Cora Linn. 

 A very fine example of a cylinder is now presented in a rock, 

 which divides the stream of the Hinter-Rhein, in the upper part 

 of the gorge of the Rofla, Canton of Grisons, We see here two 

 beautiful cascades in the middle of the river ; one falls into the 

 cylinder, the other constitutes its overflow. Large and frequent 

 segments of cylinders are seen in the high walls of chlorite- 

 slate, which form the sides of this gorge. But nowhere pro- 

 bably can the phenomenon be better seen than by walking along 

 the scaffold, which leads at the bottom of the frightful gorge of 

 the Tamina, from the Hotel at Pfeff'ers-bad to the hot-spring, 

 through a distance of more than a quarter of a mile. The chffs 

 of limestone here amount to several hundred feet in height, and 



