408 PHYSICAL GEOGRAPHY. 



weather, falling into the valley below, bringing with it detached fragments of the grit. 

 In the winter season, after unusual rains, or in severe frost, the decomposition is the most 

 rapid, the Tor discharging from its side immense pieces of its material, the noise of which 

 in their descent may be heard in the adjacent villages, and is described as singularly 

 impressive in the night. In all Alpine regions, subject to great seasonal vicissitudes, frost 

 is a powerful agent in the destruction of rocks. When the water that has entered their 

 pores and fissures becomes frozen, it acts by its expansion with irresistible force, and 

 detaches enormous masses, which fall from their parent bed thundering to a lower level. 

 In the upper parts of North America, even in latitude 51 in some places, where the 

 winter climate is so severe that brandy congeals and the lakes freeze eight feet thick, the 

 rocks split with a noise resembling the explosion of artillery, and the shattered fragments 

 fly to a considerable distance. 



The action of water, in another way, operates to dislodge from their situation the 

 higher parts of mountains, and sometimes to reduce their whole mass to ruins, producing 

 land or mountain slips. This is by a slow process of erosion and undermining, which, 

 having proceeded to a sufficient extent, brings on in a moment the catastrophe of a slide 

 or fall. The occurrence cannot take place in the case of unstratified rocks, which are only 

 subject to the gradual abrading of their entire mass, and the detachment of small frag- 

 ments ; but with reference to the stratified mountains, where layers of different kinds of 

 rock overlay one another, it is easy to conceive of such slides transpiring. Water percolating 

 by rents and fissures through an upper stratum, and reaching another which readily yields 

 to its solvent power, the lower stratum may be so far carried away in the course of ages 

 as to be unable to support the upper, which, in consequence, falls down. But little harm 

 would ensue, if the different strata were of uniform breadth and horizontally disposed, 

 like a number of equal volumes piled upon each other, instead of displaying varying 

 thickness and all manner of inclination. It is this last condition chiefly the differently 

 inclined plane upon which the upper stratum descends that causes its precipitation 

 upon the country at the base, covering it with its ruins, and occasionally overwhelming 

 its inhabitants. Other circumstances concur to the* production of land and mountain slips ; 

 but the principal agent is water, operating by a process of undermining, which, however 

 slow and subtle, is grand and terrible in the crisis that ensues. Such events are by no 

 means uncommon ; but they generally occur in secluded and uninhabited sites, so as not to 

 attract any wide notice, unless they happen upon a grand scale. On the night of the 29th 

 of January 1840, in the district of Jura, a mountain called the Carnans came down in 

 mass oh the surrounding plain, and a portion of the royal road from Dijon to Portalier 

 sunk with this eboulement to a depth of more than fifty metres. That portion known 

 as the Rampe de Carnans, the ladder or staircase of Carnans, was rendered impassable, 

 and all communication between the places on each side was entirely suspended. A fresh 

 mass of rock and earth, during the following day, was detached, and was distinctly seen 

 from a great distance as it slid down. It was supposed that a fountain, which ceased to 

 play upwards of a quarter of a century before, had then taken a new subterranean 

 direction, and mined out a portion of the mountain. Switzerland has repeatedly ex- 

 hibited these extensive falls from her giant mountains, which may form a subject of 

 interesting reference. 



It has sometimes happened that the waters of an elevated lake have insinuated them- 

 selves between the strata composing the mass of a mountain, gradually loosening and 

 removing a quantity of material, by which the superior body of rock or earth being 

 deprived of its support has fallen. In this way, the catastrophe of the Rosenburg, 

 otherwise called Mont Ruffi, is conceived to have been occasioned in the year 1806* 

 Nearly in the centre of Switzerland, in the canton of Zug, is the lake of that name, a 



