THE 



JOURNAL OF GEOLOGY 



SEPTEMBER-OCTOBER, i8g6. 



DRAINAGE MODIFICATIONS AND THEIR INTER- 

 PRETATION.' 



PART II. CRITERIA FOR DETERMINING STREAM MODI- 

 FICATIONS. 



( I ) ALIGNMENT OF THE DRAINAGE AFFECTED BY THE MIGRATION 



OF DIVIDES. 



Before attempting to apply the law of the migration of 

 divides, it is well perhaps to consider how its operation affects 

 the arrangement of the minor drainage lines ; and thus become 

 better acquainted with the criteria of change which we may 

 expect to find in the field. 



(a) Rectangular arra?igenie?it of the drainage lines. — ■ Here again 

 we must begin with the simplest conditions possible and progress 

 to the more complex. Manifestly the simplest condition we can 

 assume is that of a region so long subjected to base leveling proc- 

 esses that its surface is a plain with but slight irregularities, 

 standing at or near sea level. The strata must be practically 

 homogeneous and horizontal. The first of these conditions 

 insures an equilibrium of the streams, — a perfect inter-adjustment 

 of the branches which is impossible under other than base- 

 leveling conditions. The second eliminates the effect of geologic 

 structures and the varied character of the rocks which almost 

 always exerts an important influence. When the external influ- 

 ence of geologic structure and character of rocks is removed 



' Published by permission of the Director of the U. S. Geological Survey. 

 Vol.. IV., No. 6. 657 



