248 PRINCIPLES OF STRATIGRAPHY 



terial carried in suspension or rolled along tiie bottom of some of 

 the larger streams of the world, according to the investigations 

 of Guppy and T. Mellard Reade. 



Table sJiozving the transportation of material by rivers. 



Water in 

 Stream cu. meters 



per sec. 



Material 



carried 



in cu. meters 



per year 



Amazon 69,580 



Congo .- 50,970 



Yangtsekiang 21,810 182,000,000 



La Plata 19,820 44,000,000 



Mississippi 17,500 211,500,000 



Danube 8,502 35,540,000 



Ganges 5,762 18,030,000 



Indus 5,649 



Nile 3,680 



Huang-ho 3,285 472,500,000 



Rhine 1 ,974 



Po 1,735 11,480,000 



Pei-ho 220 2,266,000 



Thames 65 528,300 



The ability of a current to transport material varies in general 

 as the sixth power of its velocity. Thus, if the velocity is doubled, 

 the carrying power is increased 64 times. A current having a 

 velocity of 3 feet (or approximately i meter) per second (about 

 2 miles per hour) will move ordinary rock fragments of the size of 

 a hen's egg and weighing about 3 ounces. From the law of varia- 

 tion it follows that a current of ten miles per hour will carry rocks 

 weighing one and one-half tons, while a torrent of 20 miles per hour 

 will carry rock masses 100 tons in weight. Taking the varying spe- 

 cific gravities of dififerent rocks into consideration, the table on page 

 249 has been constructed by T. E. Blackwell (5; cited by Beard- 

 more-3:7; Penck-51 : ^5/) to show the size of the various rock 

 masses transported by currents of varying velocity. 



According to the experiments of Forbes {2t,:4/'4) made in a 

 shallow trough, the following velocities were required to stir up 

 various sediments from the bottom : 



Table of velocities required to stir up bottom material. 



Moist brick clay at a velocity of 0.077 iri. per sec. 



Fine fresh-water sand at a velocity of 0.213 m. per sec. 



Sea sand at a velocity of 0.337 m. per sec. 



Gravel, pea size, at a velocity of 0.610 m. per sec. 



