[25] See J.W. Gregory, "The Plan of the Earth and its Causes," _Geog. Journal_, xiii. (1899) p. 225; Lord Avebury, _ibid._ xv. (1900) p. 46; Marcel Bertrand, "Deformation tetraedrique de la terre et deplacement du pole," _Comptes rendus Acad. Sci._ (Paris, 1900), vol. cxxx. p. 449; and A. de Lapparent, _ibid._ p. 614. [26] See A.E.H. Love, "Gravitational Stability of the Earth," _Phil. Trans._ ser. A. vol. ccvii. (1907) p. 171. [27] _Rumpf_, in German, the language in which this distinction was first made. [28] _Lehrbuch der Geographie_ (Hanover and Leipzig, 1900), Bd. i. S. 245, 249. [29] See, for example, F.G. Hahn's _Insel-Studien_ (Leipzig, 1883). [30] See _Geographical Journal_, xxii. (1903) pp. 191-194. [31] The most important works on the classification of land forms are F. von Richthofen, _Fuhrer fur Forschungsreisende_ (Berlin, 1886); G. de la Noe and E. de Margerie, _Les Formes du terrain_ (Paris, 1888); and above all A. Penck, _Morphologie der Erdoberflache_ (2 vols., Stuttgart, 1894). Compare also A. de Lapparent, _Lecons de geographie physique_ (2nd ed., Paris, 1898), and W.M. Davis, _Physical Geography_ (Boston, 1899). [32] "Geomorphologie als genetische Wissenschaft," in _Report of Sixth International Geog. Congress_ (London, 1895), p. 735 (English Abstract, p. 748). [33] On this subject see J. Geikie, _Earth Sculpture_ (London, 1898); J.E. Marr, _The Scientific Study of Scenery_ (London, 1900); Sir A. Geikie, _The Scenery and Geology of Scotland_ (London, 2nd ed., 1887); Lord Avebury (Sir J. Lubbock), _The Scenery of Switzerland_ (London, 1896) and _The Scenery of England_ (London, 1902). [34] Some geographers distinguish a mountain from a hill by origin; thus Professor Seeley says "a mountain implies elevation and a hill implies denudation, but the external forms of both are often identical." _Report VI. Int. Geog. Congress_ (London, 1895), p. 751. [35] "Mountains," in _Scot. Geog. Mag._ ii. (1896) p. 145. [36] _Fuhrer fur Forschungsreisende_, pp. 652-685. [37] See, for a summary of river-action, A. Phillipson, _Studien uber Wasserscheiden_ (Leipzig, 1886); also I.C. Russell, _River Development_, (London, 1898) (published as _The Rivers of North America_, New York, 1898). [38] W.M. Davis, "The Geographical Cycle," _Geog. Journ._ xiv. (1899) p. 484. [39] A. Penck, "Potamology as a Branch of Physical Geography," _Geog. Journ._ x. (1897) p. 619. [40] See, for instance, E. Wisotzki, _Hauptfluss und Nebenfluss_ (Stettin, 1889). For practical studies see official reports on the Mississippi, Rhine, Seine, Elbe and other great rivers.