274 E. C. ANDREWS. 



its channel grade to its own strength. But until that stage 

 arrives it will slowly corrade the declivities, and with the 

 material so gained and the material otherwise transported, 

 will slowly build up the channel irregularities. 



(c) Terms "Flood " and "'Drought.'" — While on the sub- 

 ject of flood and drought action, it may be well to remove 

 any misconception that might arise from the use of the 

 term "Flood." To the popular mind the word flood is 

 associated with an ephemeral heavy rush of water or lava, 

 something in which the time factor is very insignificant. 

 In these pages a flood is supposed to have reference to 

 stream volume, without regard being had necessarily to the 

 time factor. A flood may last but the fraction of an hour, 

 or it may be drawn out for many thousands of years. A 

 period of flood action may also be taken to mean a summa- 

 tion of the action of several or of many floods of varying 

 strength. All other things being equal, an excessively 

 mobile stream will — in nature — have a flood stage period 

 of very much less duration than that of a very viscous or 

 of a stream but slightly plastic. 



In these pages, that which is known as a Glacial Period, 

 or an Ice Age, is considered as an Ice Flood. Just as the 

 periods of great volume in ordinary stream history are 

 known as their flood periods, so the periods of great ice 

 volume in the history of recent glaciation are taken as ice 

 floods. The slight fluctuations in the volumes of modern 

 glaciers, the slight movements of recession and advance 

 witnessed to-day, are analogous to the tiny fluctuations of 

 volume for ordinary streams during their low level stages. 

 But in the two flood types the time factor varies greatly, 

 in the one the flood water quickly subsides, whereas in the 

 other the period of great volume may last many thousands 

 of years. 



