Vol. I. No. 3. THE NATIONAL GEOGRAPHIC MAGAZINE. PUBLISHED BY THE NATIONAL GEOGRAPHIC SOCIETY. WASHINGTON, D. C. Price 50 Cents. CONTENTS. The Rivers and Valleys of Pennsylvania: William Morris Davis (Illustrated by one map and twenty-five cuts.) Topographic Models: Cosmos Mindeleff (Illustrated by two plates.) National Geographic Society--Abstract of Minutes International Literary Contest to be held at Madrid, Spain July, 1889. PRESS OF TUTTLE, MOREHOUSE & TAYLOR, NEW HAVEN, CONN. THE NATIONAL GEOGRAPHIC MAGAZINE. Vol. I. 1889. No. 3. THE RIVERS AND VALLEYS OF PENNSYLVANIA.[1] BY WILLIAM MORRIS DAVIS. "In Faltensystemen von sehr hohem Alter wurde die ursprüngliche Anordnung der Langenthäler durch das Ueberhandnehmen der transversalen Erosionsfurchen oft ganz und gar verwischt." LÖWL. Petermann's Mittheilungen, xxviii, 1882, 411. [Footnote 1: The substance of this essay was presented to the Society in a lecture on February 8th, 1889, but since then it has been much expanded.] CONTENTS. PART FIRST. _Introductory_. 1. Plan of work here proposed. 2. General description of the topography of Pennsylvania. 3. The drainage of Pennsylvania. 4. Previous studies of Appalachian drainage. PART SECOND. _Outline of the geological history of the region_. 5. Conditions of formation. 6. Former extension of strata to the southeast. 7. Cambro-Silurian and Permian deformations. 8. Perm-Triassic denudation. 9. Newark deposition. 10. Jurassic tilting. 11. Jura-Cretaceous denudation. 12. Tertiary elevation and denudation. 13. Later changes of level. 14. Illustrations of Pennsylvanian topography. PART THIRD. _General conception of the history of 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_.