50 A. H. PURDUE 



Fig. 2 is a map of two of these valleys with their tributaries, 

 four and a half miles north of Fayetteville. The map was made 

 with a plane-table without correction for magnetic variation. 

 These are typical of valleys that occur in the Boone chert of 

 Arkansas by the thousand. The heavy lines are placed along 

 the bottoms of the valleys only to bring to notice the angles at 

 which they join each other. These angles vary from 60 to ioo 

 degrees or more, the most common one being 80 degrees. It 

 will be noticed that the angles at which the main valleys and 

 their tributaries unite are quite different from those of corrasion. 



The writer thinks there can be no doubt but these valleys are 

 determined by the jointing of the horizontal beds of rocks. 

 The double heads which are so common among them are tribu- 

 tary valleys in their incipiency, following joints which intersect 

 along the course of the main valley. 



The large valleys which contain the master streams of the 

 Boone chert region were probably inherited, with their streams, 

 from the former superimposed rocks, but the valleys here men- 

 tioned owe their origin and development to solution, aided pos- 

 sibly by the removal of part of the residual chert after the most 

 excessive rains. A. H. Purdue. 



University of Arkansas, 

 Fayetteville, Arkansas. 



