184 National Geographic Magazine. 



Part third. General conception of the history o^ a river. 



15. The complete cycle of river life : youth, adolescence, 



maturity and old age. 



16. Mutual adjustment of river courses. 



17. Terminology of rivers changed by adjustment. 



18. Examples of adjustments. 



19. Revival of rivers by elevation and drowning by depression. 



20. Opportunity for new adjustments with revival. 



21. Antecedent and superimposed rivers. 



22. Simple, compound, composite and complex rivers. 



Part fourth. The development of the rivers of Pennsylvania. 



23. Means of distinguishing between antecedent and adjusted 



consequent rivers, 



24. Postulates of the argument, 



25. Constructional Permian topography and consequent drain- 



age, 



26. The Jura mountains homologous with the Permian Alle- 



ghanies, 



27. Development and adjustment of the Permian drainage, 



28. Lateral water-gaps near the apex of synclinal ridges. 



29. Departure of the Juniata from the Juniata-Catawissa 



syncline. 



30. Avoidance of the Broad Top basin by the Juniata head- 



waters. 



31. Reversal of larger rivers to southeast courses. 



32. Capture of the Anthracite headwaters by the growing 



Susquehanna. 



33. Present outward drainage of the Anthracite basins. 



34. Homologies of the Susquehanna and Juniata. 



35. Superimposition of the Susquehanna on two synclinal 



ridges, 



36. Evidence of superimposition in the Susquehanna tributaries. 



37. Events of the Tertiary cycle. , 



38. Tertiary adjustment of the Juniata on the Medina anti- 



clines. 



39. Migration of the Atlantic-Ohio divide, 



40. Other examples of adjustments. 



41. Events of the Quaternary cycle. 



42. Doubtful cases. 



43. Complicated history of our actual rivers. 



44. Provisional conclusions. 



Part first. Introductory. 



1. Plan of work here proposed. — l^o one now regards a river 

 and its valley as ready-made features of the earth's surface. All 

 are convinced that rivers have come to be what they are by slow 

 processes of natural development, in which every peculiarity of 

 river-course and valley-form has its appropriate cause. Being 



