Description of Obtuse Conical Alluvium. 13 



perpendicular walls of alpine gorges must in time give way 

 from the same cause, the erosion of their base. In all the gra- 

 dations from the hardest granite to the most yielding sand, it 

 will follow, that the more durable the banks of the channel, the 

 more deeply must that channel be cut before they will collapse ; 

 and hence in all valleys of erosion, as a general rule, the widtli 

 of the valley will exceed its depth, in proportion to the softness 

 of the materials composing its sides. One river meanders in an 

 expanded vale between banks of clay, sand, or gravel, a few 

 feet in height ; another flows at the bottom of a deeper valley 

 between cliffs of chalk or sandstone ; and a third at the base of 

 precipitous mountains, where it is often concealed from sight 

 between lofty walls of the older rocks. 



The inchnation of any mass of sediment is found to correspond 

 to the inchnation of the current, by which it is deposited. Of 

 the forms of alluvium resulting from this law, one of the most 

 striking is the Obtuse Cone, which is to be seen in every alpine 

 valley, where streams enter it through ravines or smaller lateral 

 and more elevated valleys. The same form also frequently pre- 

 sents itself on the margins of lakes at the termination of such 

 ravines or valleys. (See the Sketch, Fig. 9.) >r»p)dJiW 



The obtuse cone makes an angle with the horizon of from 5° 

 to 15^ In many cases its apex is not less than 500 feet higher 

 than its base, and its diameter 3 or 4 miles. It is distinguished 

 from the acute cone, not only by the obvious difference of form, 

 but by the circumstance that the largest fragments remain at the 

 top of the cone, and the finest are washed to the bottom ; where- 

 as, in the acute cone, the reverse arrangement takes place. 

 ,, Every obtuse cone, in the present state of the Alps, exhibits 

 several vaiieties of surface. A space, enclosed by two radii pro- 

 ceeding from the apex, serves as the bed of the torrent. It ex- 

 hibits a sloping surface, consisting entirely of boulder-stones and 

 coarse gravel, over which the current takes various directions, 

 and continues to deposit its solid contents. Another large por- 

 tion, especially the higher part, consists of similar rough mate- 

 rials, but is covered either with a forest of firs, or with alders and 

 other coppice-wood. Other parts are destitute of trees, and not 

 more productive than the stony flanks of high mountains usually 



