THE WORK OF RUNNING WATER. 



105 



would be great. The complete and final diversion of the stream through 

 the new channel might be a slow process. 



Piracy may occur where the material in which the valleys are cut 

 is homogeneous; but, as will be 

 seen later, heterogeneity of mate- 

 rial, by determining unequal 

 rates of erosion, stimulates the 

 piratical proclivities of streams. 



An actual case of piracy is 

 shown on Plate XL North and 

 South Lakes formerly drained 

 westward to the Schoharie 

 Creek, the present head of 

 which is in the extreme north- 

 west corner of the map. The 

 head of Kaaterskill Creek, which 



had a much higher gradient, Figs. 92 and 93.— Domestic piracy. The 

 worked back and captured the tributary, a of Fig. 92, develops headward 



head of the westward-flowing ^"^^^ '^ |^^Pf^ *^^ ."^^^^ ^^^^^"^ ^t ^' s^^^i^g 



-. . , , . the result shown in Fig. 93. 



stream, divertmg the dramage 



from North and South Lakes to itself. Schoharie Creek was thus 



beheaded. 



Plaatekill Creek, near the south limit of the map, appears to have 

 beheaded the creek flowing west and northwest, similarly diverting its 

 head waters. The Dells, Wis., quadrangle (U. S. Geol. Surv.) affords 

 an illustration of domestic piracy. 



RATE OF DEGRADATION. 



The amount of mechanical sediment which the Mississippi River 

 carries to the Gulf of Mexico is estimated to represent a rate of degra- 

 dation for the Mississippi basin of about one foot in 5000 years. But 

 the mechanical sediment carried to the Gulf does not really represent 

 the total degradation of the basin, for the water which sinks beneath 

 the surface is dissolving more or less rock substance, especially lime 

 carbonate. This material is carried to the sea in solution, and does 

 not appear in the sediment on which the above estimate is based. 

 Taking into account the matter dissolved by the water and carried to 



