23. Means of distinguishing between antecedent and adjusted consequent rivers. 24. Postulates of the argument. 25. Constructional Permian topography and consequent drainage. 26. The Jura mountains homologous with the Permian Alleghanies. 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 headwaters. 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 anticlines. 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_.--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 fully persuaded of the gradual and systematic evolution of topographic forms, it is now desired, in studying the rivers and valleys of Pennsylvania, to seek the causes of the location of the streams in their present courses; to go back if possible to the early date when central Pennsylvania was first raised above the sea and trace the development of the several river systems then implanted upon it from their ancient beginning to the present time. The existing topography and drainage system of the State will first be briefly described. We must next inquire into the geological structure of the region, follow at least in a general way the deformations and changes of attitude and altitude that it has suffered, and consider the amount of denudation that has been accomplished on its surface. We must at the same time bear in mind the natural history of rivers, their morphology and development; we must recognize the varying activities of a river in its youth and old age, the adjustments of its adolescence and maturity, and the revival of its decrepit powers when the land that it drains is elevated and it enters a new cycle of life. Finally we shall attempt to follow out the development of the rivers of Pennsylvania by applying the general principles of river history to the special case of Pennsylvania structure.