TOPOGRAPHY. 81 



system gradation plains and present stream beds are more nearly coinci- 

 dent, but on the lower Allegheny, the Monongahela, the tributaries of 

 these streams, the Ohio, and its southern, and smaller northern, as well as 

 the lower courses of its large northern, tributaries the gradation plains stand 

 above the present streams. The Ohio and Allegheny, as shown below, 

 have been formed by the combination of several drainage systems which 

 were more or less independent. In accord with and in support of this 

 interpretation the slope of the gradation plains is found to be out of har- 

 mony with the present Allegheny-Ohio system, btit in harmony with 

 certain ancient systems whose extent and connections are outlined below. 



MON XLI 6 



