G. E. MITCHELL. Landslides and Rock Avalanches, Nat. Geogr. Mag., vol. 21, 1910, pp. 277-287. WILLIAM H. HOBBS. Soil Stripes in Cold Humid Regions and a Kindred Phenomenon, 12th Rept. Mich. Acad. Sci., 1910, pp. 51-53, pls. 1-2. Relation of deforestation to erosion:— N. S. SHALER. Origin and Nature of Soils, 12th Ann. Rept. U. S. Geol. Surv., 1891, Pt. 1, pp. 268-287. W. J. MCGEE. The Lafayette Formation, _ibid._, pp. 430-448. F. H. KING. Soils. Macmillan, New York, 1908, pp. 50-54. BAILEY WILLIS. Water Circulation and Its Control, Rept. Nat. Conserv. Com., 1909, vol. 2, pp. 687-710. W. J. MCGEE. Soil erosion, Bull. 71, U. S. Bureau of Soils, 1911, pp. 60, pls. 33. CHAPTER XII THE LIFE HISTORIES OF RIVERS =The intricate pattern of river etchings.=—The attack of the weather upon the solid lithosphere destroys the integrity of its surface layer, and through reducing it to rock débris makes it the natural prey of any agent competent to carry it along the surface. We have seen how, for short distances, gravity unaided may pile up the débris in accumulations of talus, and how, when assisted by thaw water which has soaked into the material, it may accomplish a slow migration by a peculiar type of soil flow. Yet far more potent transporting agencies are at work, and of these the one of first importance is running water. Only in the hearts of great deserts or in the equally remote white deserts of the polar regions is the sound of its murmurings never heard. Every other part of the earth’s surface has at some time its running water coursing in valleys which it has itself etched into the surface. It is this etching out of the continents in an intricate pattern of anastomosing valleys which constitutes the chief difference between the land surface and the relatively even floor of the oceans. =The motive power of rivers.=—Every river is born in throes of Mother Earth by which the land is uplifted and left at a higher level than it was before. It is the difference of elevation thus brought about between separated portions of the land areas that makes it possible for the water which falls upon the higher portions to descend by gravity to the lower. This natural “head” due to differences of elevation is the motive power of the local streams, and for each increase in elevation there is an immediate response in renewed vigor of the streams. The elevated area off which the rivers flow is here termed an upland.