532 STEVENSON— THE FORMATION OF COAL BEDS. [Nov. i, 



the fan. These usually fall from the mountains, and the torrents,, 

 no matter how strong, cannot carry them far. The fine mud and 

 sand are not deposited along the torrents but are carried out by the 

 rivers to become fertile alluvium. He describes dejection cones 

 made by the Adour, Garonne and other streams, which have become 

 confluent and which are now gashed by diverging currents. In 

 discussing modes of protection against devastation by torrents, 

 Surell says that it is unnecessary to wait until the region has be- 

 come reforested. " It suffices if the surface be carpeted with grass, 

 brush or shrubs. The herbaceous plants and the brush, as com- 

 pletely as the trees, protect the surface of the soil, divide the 

 streams which tend to ravine it, prevent abrupt concentration of the 

 waters and absorb a certain portion in the spongy humus, which has 

 formed at their foot." He devotes several pages to discussion of 

 this topic and gives a long list of plants which take possession of 

 devastated areas, some of them growing on naked rock. 



One who examines only the illustrating figures given in text- 

 books is in danger of supposing that alluvial fans are of limited 

 extent, confined mostly to comparatively narrow river-valleys with 

 abrupt sides; but the conception would be erroneous. Gras,^-* in 

 writing of alpine diluvium in southeastern France, says that a great 

 area between the Rhone and the first calcareous mountain of the 

 Alps is filled with clay, sand and pebbles. This thick mass extends 

 northward to beyond Dijon and the Saone Valley and follows the 

 Rhone southward to the Mediterranean shore. The chief develop- 

 ment is in the Departement of Isere, whence it becomes thinner 

 southward. He recognizes a vast dejection cone, or, better, alluvial 

 fan on the Dauphiny plain, whose summit is in the Grand Chartreuse 

 chain and whose base has a radius of 70 to 75 kilometers. The ma- 

 terials came from the mountains at the east and contain the charac- 

 teristic rocks of Mount Blanc and other areas, so that they have 

 been transported far. The streams have heaped up pebbles to the 

 thickness of hundreds of meters. 



"* Sc. Gras, " Sur la periode quaternaire, dans le vallee du Rhone et sa 

 division en cinq epoques distinctes." Bull. Soc. Geol. dc France. II.. Vol. 

 XIV., 1857, p. 207. 



