30 Remarks on the Formation of Alluvial Deposites. 



fluence can bear no comparison with the vast effects which it 

 must have produced before population was extended, before 

 man was created, and while this same agency was preparing the 

 bare and desolate lands for sustaining the growth of plants, 

 and was thus gradually rendering them capable of their present 

 methods of conservation. 



III. — Of Detritus conveyed by Running into Standing Water. 



Whenever a stream, charged with detritus, meets standing 

 water, a separation takes place between those parts of the detri- 

 tus Avhich are sufficiently fine to be held in suspension bj^ the 

 stream, and those which it pushes or rolls along the surface of 

 the ground. The stream, diffusing itself over the standing wa- 

 ter, until its impetus is destroyed by the resistance, carries with 

 it the fine particles, and lets them fall slowly in clouds to the 

 bottom ; whereas the larger and heavier particles, the moment 

 they reach the standing water, fall by their own weight, and 

 assume the two forms described in the former part of this pa- 

 per, i. e. according to circumstances, cither the lengthened talus 

 or the acute cone, the upper part of each form being, however, 

 modified by the water flowing over it. 



An accidental occurrence afforded me, last summer, an op- 

 portunity of observing this process as distinctly as if an experi- 

 ment had been contrived on purpose to illustrate it. Among 

 other works recently executed on the Birmingham Canal, there 

 bad been a deep cutting through a bank, which consists of a 

 mixture of sand, clay, and pebbles, and thus a fresh smooth 

 slope a b. Fig. 22, was exposed to the action of the atmosphe- 

 ric waters. At the bottom of this sloping bank, and between it 

 and the towing-path, there was at intervals a slight depression, 

 which held the water, so as to form small pools at the foot of 

 the bank. There had been very heavy rains a day or two be- 

 fore, and the bank bore traces of their influence in the nume- 

 rous furrows upon its surface. The water having sunk through 

 the ground, the pools were left dry. There remained at the 

 bottom y a thin coat of glistening slime or mud, and along the 

 foot of the bank an even terrace, the upper slope of which h c 

 was nearly flat, whereas the lower slope c d was as highly in- 



