18 Remarks on the Formation of' Alluvial Depositcs. 



small scale, may enable us to account for similar results, the 

 steps of which necessarily escape our observation. 



Among the pebbles, or other pieces of stone, carried in a 

 stream, some will, in consequence of their size, weight, and form, 

 be arrested and detained by inequalities in the bed of the stream. 

 Thus a flat stone is, for a while, carried swiftly along, moving 

 on its circumference like a quoit, but continually vacillating 

 with the varying impressions of the water. At length it im- 

 pinges against some projection in the bedj which turns its broad 

 surface against the stream, and there, if the projection be strong 

 enough to retain it, it will remain. (See Fig. 14.) Other stones, 

 whether flat, or approaching more to an oval or spherical form, 

 are often wedged between those above and below them, (Fig. 

 15) ; and a few strong points of support, thus provided, form 

 the basis of a natural dam or weir. Hence arise the numerous 

 minor waterfalls or rapids, which occur in rivers flowing over 

 beds of detritus, no less than in those which pass over rocks in 

 place. Pebbles and finer materials fill the hollow behind the 

 dam, as fast as it rises, until the bed of the river above it be- 

 comes perfectly level. 'J'he ledge, thus produced, often crosses 

 the bed of the stream in a devious and serpentine form, because 

 the lateral action of the banks, or of inequalities in the bed, 

 forces portions of the stream from its direct course, and thus 

 hurls the stones sideways as well as downwards. 



So long as the flood which has deposited these materials con- 

 tinues unabated, the dam and the level above it will extend un- 

 interruptedly across the river. It is remarkable that these firm 

 dykes are demolished, not by the increase, but by the subsidence 

 of the stream ; and tiie action appears to be as follows. During 

 a copious flood, the whole is covered by a mass of flowing wa- 

 ter, so that each stone, being surrounded with water pressing it 

 in all directions, remains in its place. But, when the flood sub- 

 sides, the pressure is only from behind. The whole mass of 

 stones and gravel being fully charged with water, that fluid 

 drives before it certain portions, which, though fixed with sufB>- 

 cient firmness to keep their places, while aided by a pressure in 

 front, give way as soon as that is removed. The removal of 

 some masses occasions the removal of others which rest upon 

 them, and all the waters now drain themselves through the 



