Temporary Lakes Jhrmed by Obtuse Cones. 1 5 



of water so much exceeds the quantity of debris as to carry 

 nearly the whole of it away, a cascade remains, as is represented 

 in Fig. 4, and seen in the well-known example of the Pisse- 

 vache. But if the quantity of solid matter is too great for the 

 water, an obtuse cone is formed with its apex at the mouth of 

 the ravine. If the torrent has yielding materials to work upon, 

 the quantity brought down by it is often so great as to make it 

 appear rather a solid than a fluid substance, and to overspread 

 the surface of the cone to an enormous depth. Hence the Alp- 

 bach above Meiringen, in the Canton of Berne, has twice proved 

 nearly fatal to that village by burying it in " lias-marl,"" not- 

 withstanding the vast mounds of stone, diverging from the apex, 

 which were built nearly a century ago to restrain its ravages. 

 A village near Aigue-belle, in Savoy, was enveloped by the same 

 process in 1752, so that only the tower of the church was left 

 rising above the sediment, which formed a stratum 15 or 20 

 feet thick *. 



The obtuse cone often has the effect of obstructing the course 

 of a river, so as to produce inundations. The abundance of 

 earth and stones brought down by the lateral stream is occasion- 

 ally so great as to hem in the principal stream, which thus forms 

 a lake above the dam. But from the nature of the materials, 

 the obstruction is easily removed ; the principal stream swells 

 again, asserts its pre-eminence, rapidly tears away the base of the 

 encroaching cone, and sinks again to its former level. The 

 effect of this abrasion of obtuse cones by the principal stream 

 is seen in planes cutting across them near the base, and inclining 

 at an angle of about 45° to the horizon. These steep declivi- 

 ties, if we may judge from the forest trees growing upon them, 

 are often not less than 100 feet high. They occur more fre- 

 quently in proportion to the narrowness of the valley. In the 

 Linth-thal, which descends with uncommon rapidity, and has nu- 

 merous lateral valleys, the cones occur so frequently as to en- 



• De I.uc, Lettres sur I'Hist. de la Terre, t. ii. p. 75 I have found De Luc's 



account of the alluvial and disintegrating processes in the 30th and 31st letters 

 remarkably correct and interesting. He designates the form of alluvium, 

 which I am describing, by the general term cone, and says, p. 67, that it has 

 the shape « d'un pain de sucre fort applati, coupe par son milien du sommet 

 a la base." 1 



