20 IOWA STUDIES IN NATURAL HISTORY 



Fis. 6. A plat of the drainage of the Pittsbura- Quadrannle. There are two distinct 

 sets of streams, one including- the large and most crooked streams, and the other the 

 smaller and straiffhter ones. The lar.e.'e streams have a sort of curve different from 

 that of the small ones. It seems that the laise streams must have been developed to 

 something like their present sizes and must have had their present courses before the 

 small streams were started. The interpretation seems warranted that the courses of 

 the large streams were established in old age of the first cycle of erosion and the 

 small streams in the second cycle. 



