Page:The National geographic magazine, volume 1.djvu/240

 General conception of the history of a river.

The complete cycle of river life: youth, adolescence, maturity and old age. Mutual adjustment of river courses. Terminology of rivers changed by adjustment. Examples of adjustments. Revival of rivers by elevation and drowning by depression. Opportunity for new adjustments with revival. Antecedent and superimposed rivers. Simple, compound, composite and complex rivers.

The development of the rivers of Pennsylvania.

Means of distinguishing between antecedent and adjusted consequent rivers. Postulates of the argument. Constructional Permian topography and consequent drainage. The Jura mountains homologous with the Permian Alleghanies. Development and adjustment of the Permian drainage. Lateral water-gaps near the apex of synclinal ridges. Departure of the Juniata from the Juniata-Catawissa syncline. Avoidance of the Broad Top basin by the Juniata headwaters. Reversal of larger rivers to southeast courses. Capture of the Anthracite headwaters by the growing Susquehanna. Present outward drainage of the Anthracite basins. Homologies of the Susquehanna and Juniata. Superimposition of the Susquehanna on two synclinal ridges. Evidence of superimposition in the Susquehanna tributaries. Events of the Tertiary cycle. Tertiary adjustment of the Juniata on the Medina anticlines. Migration of the Atlantic-Ohio divide. Other examples of adjustments. Events of the Quaternary cycle. Doubtful cases. Complicated history of our actual rivers. Provisional conclusions.

1. Plan of work here proposed.—No 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