WATER AS A MECHANICAL AGENT. 177 



The floods of the Nile commence in southern Abyssinia (where the 

 annual fall of rain is 50 inches or more) in April, and reach Cairo in mid- 

 sn miner, and exert their beneficial influence over all the flood grounds 

 by the fertile silt deposited, which is estimated to amount annually to 

 140 millions of tons. The maximum rise is 40 feet, and the area of the 

 region flooded is 2100 square miles. 



The distribution of tributaries influences the time and amount of floods. 

 In the Amazon, the tributaries north of the equator are flooded during the 

 xaiuy season of the northern hemisphere, and those south, during that of the 

 southern. In this way many rivers, by their widespread arms, take advan- 

 tage of the differences in the seasons or climates of the distant countries 

 whence they get their supplies. The floods of the Amazon convert the larger 

 part of its 500,000 miles of silvas into one great lake ; 3000 miles up the river, 

 a,n elevation abore tide of only 210 feet is reached. The Mississippi hardly 

 feels the great floods of the Ohio unless they come when the Rocky 

 Mountain tributaries are also flooded; and these western tributaries are so 

 widely distributed and so large that they may make successive floods, or pour 

 in all together in one vast deluge, giving the Mississippi in some places 

 below the Ohio a breadth of 50 miles. At high water the flood-level is 70 

 feet above low water at Cincinnati, 51 on the Mississippi at Cairo, and 17 at 

 New Orleans. 



The cycles of rainy and dry seasons sometimes seem to correspond with 

 the sun-spot cycle of 11 years ; and greater cycles include 4 or 5 of the 11- 

 year cycles. No definite conclusions have as yet been formed regarding this 

 point. 



5. Causes tending to determine the direction of draining courses. — The 

 chief causes are the following. As regards, — 



(a) Slope. — The steepest descent accessible. 



(6) Surface-form. — A depression leading downward to concentrate the 

 waters from a large area for work. 



(c) Basement rocks. — The belt of least resistance to wear. In the case 

 of upturned strata, whether folded or in monoclines, the belt of weaker 

 rock in the line of strike ; or over folded rocks, the course of a region of 

 warped strata between the extremities, overlapping or not, of the folds (page 

 388). 



{d) Fractures, faults. — The courses of great fractures and faults, and 

 especially those attending the flexing of rocks in mountain-making, as, for 

 example, those which determined the location of the Great Appalachian 

 valley of eastern Tennessee and its continuation northeastward (page 356). 



(e) Meteorological conditions. — The belt or region of greatest precipi- 

 tation. 



Denudation. 



1. WorTc of the rain-drop. — Denudation by simple impact of water com- 

 mences with the descending rain-drop. The drop makes a shallow impres- 

 dana's manual — 12 



