Account of Landslips. 3 



are exposed to the attrition of simple water, prove that its ac- 

 tion, where perceptible in any degree, is exceedingly slow, and 

 its effects inconsiderable. But very different are the appearances 

 exhibited, when a stream of water is charged with fragments of 

 rock, previously loosened and thrown within its reach. Such 

 fragments not only impel and batter one another with tremen- 

 dous fury, but shake, loosen, and separate their kindred rocks, 

 which are in place, and destroy, to a much greater extent, the 

 softer strata, to which they are carried. Hence the separation 

 of fragments of rock by agents, distinct from flowing water, 

 requires to be considered as the first step towards the formation 

 of alluvium. 



The processes of separation are of two kinds ; in the one case, 

 great masses are detached suddenly, in the other, the progress 

 of disintegration is slow, gradual, and constant. 



1. The sudden separation of a great mass is often called the 

 fall of a mountain : but the term Landslip appears more ap- 

 propriate. A slice from the side of a mountain is all that really 

 falls.* 



In all mountainous countries, which are subject to earth- 

 quakes, and in Switzerland among the rest, these events are 

 sometimes attended by the sudden fall of great masses of earth 

 and rock. Thus an earthquake is said to have detached parts 

 of the mountain of Glarnisch, Canton of Glarus, in the year 

 1593. But such instances are very rare. 



Landslips commonly take place, not in primitive mountains, 

 but in the more recent and distinctly stratified formations, 

 whether calcareous or sedimentary. The rocky strata of these 

 formations are occasionally separated by a bed of clay, and still 

 more frequently by a yielding shale or sandstone. Water, 

 slowly insinuating into the clay, converts it into mud, or it gra- 

 dually carries away portions of the soft shale or tender sand- 

 stone. When this has been done, the superincumbent stratum 

 of rock, if destitute of support underneath, slips down, resolving 

 itself into innumerable fragments. It is accompanied in its fall 

 by rocks, woods, fields, houses, and whatsoever else rests upon 



• See De la Beche's Sections and Views, Plate 33. " Fall of the Rossberg 

 (conglomerate) ; also PI. 35. fig. 3. Pinhay Cliffs, Lyme Regis (chalk) ; and 

 PI. 38. fig. 5. Diablerets (limestone). 



