276 TRANSPOETATION OF ROCK DEBRIS 



Geikie,^ the capacity o£ transport increases as the sixth power 

 of the velocity of the current; that is to say, the motor power 

 is increased sixty-four timeSy by doubling the velocity. The 

 following table is taken from the work quoted as showing the 

 power of transport of river currents of varying velocities : 



Inches Miles 



PER Sec. per Hb. 



3 0.170 : will jnst move fine clay. 



6 0.240 : will lift fine sand. 



8 0.4545: will lift sand as coarse as linseed. 



12 0.6819: will sweep along fine gravel. 



24 1.3638: will roll along rounded pebbles 1 ineli in diameter. 



36 2.045 : will sweep along slippery, angular stones of the size 



of an egg. 



There are, of course, other factors that should be taken into 

 consideration, such as the character of a river bed, the density 

 of the water, etc., but which lack of space prevents our touch- 

 ing upon here, and which are, moreover, sufficiently enlarged 

 npon In other 'works. 



The writer has stood at the head waters of the Missouri, and 

 seen the JeflEerson, Madison, and Gallatin rivers uniting their 

 floods to form one grand rushing stream of clear green water, 

 full of trout and grayling. He has seen it again at Mandan, 

 Dakota, a sluggish stream actually yellow with suspended silt. 

 At St. Louis it forms a mighty torrent, whirling along trunks 

 and stumps of trees, twigs, and all manner of organic debris 

 and inorganic detritus picked up from its banks, or washed in 

 by rains and tributary streams, till, one vast sea of liquid mud, 

 it pours every year into the Gulf of Mexico a mass of sediment 

 equal to 812,500,000,000,000 pounds (7,468,694,400 cubic feet), 

 or enough to cover a square mile of territory to a depth of 268 

 feet. But only a portion of the detritus carried by running 

 streams reaches the ocean; otherwise little attention need here 

 be given to its consideration. Nearly all streams, in some part 

 of their courses, flow through level plains with low banks which 

 are subject to inundation during seasons of high water. Con- 

 sider, then, a muddy stream such as is shown in cross-section 

 in Fig. 25, and which at ordinary periods is confined within 

 the narrow channel near the centre. In time of freshet, however, 

 the volume of water is so greatly augmented as to cause it to 



^Text-book of Geology, 3d ed. 



