CORRASION BY GRAVITY STREAMS. 275 



In the flood and drought stages of streams we have a 

 contrasting of the time and volume factors whose values 

 are not simply related. Increase of volume means increase 

 of velocity and therefore increase of corrasive power in 

 geometrical progression, while increase of time simply 

 means a simple increase of the corrasive work accomplished. 



When in previous reports [Andrews (a) and (b)] the belief 

 has been declared that, in nature, stream channel forms 

 are the result of flood action and that the interflood stream 

 is a hopeless incompetent, the meaning was simply that 

 the time factor during the interflood period of stream cor- 

 rasion has been altogether outmatched by the rapid 

 increase of corrasive power attendant on increase of volume. 



Let us see if light may not be thrown on this distinction 

 between absolute and comparative corrasive work of 

 streams by a comparison with ordinary business methods. 

 For example, a penny is in reality a very useful coin, with 

 it a large daily newspaper may be purchased, or a long ride 

 taken on a street car. Notwithstanding the decided 

 value of such a coin no business firm would entertain the 

 idea of exacting the odd penny from a customer whose 

 purchase amounted, say, to £100 6s. Id. Even for a pur- 

 chase of £5 value, the seller often gives some small article 

 in as not affecting the general amount. Again no con- 

 tractor offers a house for sale at £859 10s. Id. or £999 

 19s. llfd. In fact he takes his unit of price as not less 

 than £5. He may offer a dwelling for £725 or £1,000, 

 but not for £726 or £998. 



In a similar sense we say that the flood stream does all 

 the work of corrasion, and that the normal or drought 

 stream is a hopeless incompetent, for while yet such 

 enfeebled stream action has not, apparently, in slight 

 degree even, affected the grand enveloping profiles, a flood 

 stage again sets in and carries on its own profile making. 



R— Nov. 3, 1909. 



