DEFINITION OF A TORRENT 149 



every kind resulting from superficial erosion. The black schists dis- 

 integrate in small fragments and form small ravines analogous to those 

 in the granites and in the mica schists of the C^vennes and of the Central 

 Plateau. The other "black soils" are even more easily washed and 

 erode with very great rapidity. Soils of this kind are deeply ravined 

 as soon as they are denuded. Glacial deposits also erode with extreme 

 ease. These muds, often soaked to a great depth by rain or by melted 

 snow, flow wherever they are not held in place by vegetation. The 

 ravines that they make deepen very rapidly and become the courses of 

 torrents which transport the detritus into the valleys below. It often 

 happens that erodible soil rests on a steep, rocky, or compact clayey 

 bed. Here the water filters into the top soil and great masses of earth 

 are detached and slide to the bottoms of the ravines. Unstable ground 

 frequently flows in a rock-mud-water lava. Fragments of all kinds of 

 rock accumulate in the ravines and mix with the eroded earth from the 

 black soil, forming a fluid mass which shdes slowly or rapidly according 

 to the slope of the stream bed and the amount of rain. 



In the Cevennes and the Central Plateau the slopes formed by gneiss, 

 mica schists, Paleozoic schists, and granites disintegrate the most easily. 

 True torrent gorges are not formed on them, but instead a multitude of 

 furrows and ravines, which transport great quantities of sandy material 

 and fragments of rocks. It is a region of torrential rivers rather than of 

 torrent courses such as are found in the Alps. The Pyrenees are char- 

 acterized by an abundance of glacial or semi-glacial deposits. The 

 granites disintegrate and the detritus covers considerable areas. In 

 the Corbieres the marls are especially exposed to erosion, which form 

 short, straight torrent gorges or ravines which have numerous branches 

 that feed and fill up the water courses with detritus. 



Limestone is usually fissured, so that rain water rapidly penetrates 

 the interior of the rock if the surface is not protected by thick grass or 

 by well-rooted forest vegetation. Where the soil becomes denuded 

 steady and deep erosion often forms gorges that have abrupt slopes. 

 It is clear that excess of water is the chief danger on all easily erodible 

 soils. From the forester's standpoint, however, too little water or 

 drought is the greatest obstacle to the reclamation of land of this type, 

 for the soU becomes baked and excessively arid as soon as it loses its 

 protective vegetative cover. The soils thus suffer from the extremes of 

 too much moisture and lack of moisture. 



Definition of a Torrent. — The snow on the high mountains protects 

 the rocks and soil against various disintegrating agencies but not against 

 erosion due to glaciers and to subglacial water. Erosion of the soil 

 and of the rocks accordingly takes place below the limit of perpetual 

 snow, a limit that ranges from 8,850 to 10,800 feet in the Alps and in 



