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(a) the lengthened talus, which in general covers the base of all 

 calcareous, and conglomerate or sedimentary rocks ; and (6) the 

 acute cone, which is discharged from the ravines of highly inclined 

 schistose rocks, having a cleavage which meets the planes of stra- 

 tification at an acute angle. 



II. The materials thus furnished are distributed by streams, 



which round off their angles by continual friction, so as to convert 

 them into pebbles, sand, and mud. The hard and heavy fragments 

 driven along by streams, also wear down the rocks in place, the 

 latter being acted upon according to their degrees of softness and 

 their proneness to disintegration. 



When the detritus thus produced is discharged from a lateral 

 into a principal ravine, or valley, the divergence of the stream gives 

 it the form of a cone ; but as the force of running water carries 

 loose materials much further than they would fall by their own 

 weight, the form thus produced is not an acute but an obtuse cone. 

 In the Alps some of these obtuse cones attain 500 feet in height, 

 and three miles in diameter, bearing upon their surfaces forests 

 and villages. 



The quantity of solid materials descending over the apex of an 

 obtuse cone, is sometimes so great as to stop up the valley. The 

 waters of the principal stream then accumulate above the obstruc- 

 tion, and after the subsidence of the lateral stream, tear away the 

 base of the encroaching cone. This form the author designates 

 as the obtuse cone dipt at the base. 



Narrow valleys and plains are frequently divided by transverse 

 ledges of gravel. The formation of these is attributed to the opera- 

 tion of rivers, which it is supposed had first accumulated their de- 

 tritus in dams, and that these dams, having been successively broken 

 down after the subsidence of floods, were re-produced upon a rise 

 of the streams. 



Numerous causes are assigned which vary the depth of streams. 

 These are, rains ; the melting of Alpine snows and glaciers ; the 

 breaking up of ice in rivers; and the bursting of lakes. 



III. — Whenever detritus is conveyed by running into standing 

 water, a separation takes place between those finer particles which 

 are held in suspension, and those which it only rolls along the 

 bottom. 



As the debris of horizontally stratified rocks forms a length- 

 ened talus at their base, so the loose and heavy materials washed 

 down the side of a mountain, and conveyed into a lake, as soon as 

 they reach its margin fall in a steep slope of the same description. 

 Layer after layer is thus deposited, the result of which is, that a 

 terrace is gradually formed, dipping under the surface of the lake 

 with a gentle slope, and then abruptly terminating in a steep de- 

 clivity. 



The author next endeavours to show, that what is commonly 

 called a Delta is more strictly speaking the Sector of a Circle. 



After describing numerous examples of forms of alluvial matter, in 

 artificial reservoirs and in lakes, the author alludes to the probable 



