Aqueous and Atmospheric Erosions. 295 



the same mass greater than their inclination, while their mass 

 is considerably less. 4. A maximum velocity in the regular 

 course of many rivers in transverse valleys is frequently be- 

 tween 7 and 11 Paris feet per second. Their velocity, how- 

 ever, is at other places so considerable, that they have 

 always force sufficient to move small shingle. 5. The quan- 

 tity of matter held in suspension in glacier-brooks and all 

 Alpine streams is usually very great, and exceedingly in- 

 creases their eroding power. 6. By erosive action the bed 

 of a river may be very deeply excavated in the hard rock : 

 such channels reach their utmost development in the more 

 inclined ravines ; they remain, however, confined to the 

 valley-bottom, and have no important influence on the forma- 

 tion of the ravine itself. 7. The sudden evacuation of vast 

 reservoirs of water participates very considerably in the phe- 

 nomena of erosion and transport of rocks. Owing to the 

 velocity and power of these floods, it results that the volume 

 of rushing water is far surpassed by the mass of rocky ma- 

 terial washed down and deposited about at difl^erent places. 

 8. The formation of earthy detrital matter (Erdkrume) by 

 mechanical disintegration and chemical decomposition of 

 rocks, proceeds rapidly at the highest summits. Its accu- 

 mulation, however, and the covering up of neighbouring flat 

 areas, are prevented by the steep declivities and the isolated 

 situation of such points. 9. Vegetation is always highly 

 essential for the fixing of earthy matter on the inclined sides 

 of mountains ; hence, at great heights, and in the absence of 

 the growth of grass, the occurrence of humus, even in 

 slightly inclined spots, is but very occasional and isolated. 



10. In the Alps, particularly in the case of the crystalline 

 slate rocks, the composition of the earth and its physical 

 properties are very favourable to vegetation. Its proportion 

 of humus is very considerable, even at great altitudes. 



11. The glaciers not only aid in producing superficial changes 

 by the transport of their moraine-masses, but also by giving 

 rise to an immense quantity of fine sand, which can usually 

 be carried far away by the rivers. And, lastly, 12. The 

 loosening of great masses of rock by the weather and water 

 cause vast land-and-mountain-slip» ; and the streams tra- 

 versing the bottoms of the longitudinal valleys, owing to 



