It would be impracticable to go fully into the varieties of each specific form; but, partly as an example of modern geographical classification, partly because of the exceptional importance of mountains amongst the features of the land, one exception may be made. The classification of mountains into types has usually had regard rather to geological structure than to external form, so that some geologists would even apply the name of a mountain range to a region not distinguished by relief from the rest of the country if it bear geological evidence of having once been a true range. A mountain may be described (it cannot be defined) as an elevated region of irregular surface rising comparatively abruptly from lower ground. The actual elevation of a summit above sea-level does not necessarily affect its mountainous character; a gentle eminence, for instance, rising a few hundred feet above a tableland, even if at an elevation of say 15,000 ft., could only be called a hill.[34] But it may be said that any abrupt slope of 2000 ft. or more in vertical height may justly be called a mountain, while abrupt slopes of lesser height may be called hills. Existing classifications, however, do not take account of any difference in kind between mountain and hills, although it is common in the German language to speak of _Hugelland_, _Mittelgebirge_ and _Hochgebirge_ with a definite significance. The simple classification employed by Professor James Geikie[35] into mountains of accumulation, mountains of elevation and mountains of circumdenudation, is not considered sufficiently thorough by German geographers, who, following Richthofen, generally adopt a classification dependent on six primary divisions, each of which is subdivided. The terms employed, especially for the subdivisions, cannot be easily translated into other languages, and the English equivalents in the following table are only put forward tentatively:-- RICHTHOFEN'S CLASSIFICATION OF MOUNTAINS[36] I. _Tektonische Gebirge_--Tectonic mountains. (a) _Bruchgebirge oder Schollengebirge_--Block mountains. 1. _Einseitige Schollengebirge oder Schollenrandgebirge_-- Scarp or tilted block mountains. (i.) _Tafelscholle_--Table blocks. (ii.) _Abrasionsscholle_--Abraded blocks. (iii.) _Transgressionsscholle_--Blocks of unconformable strata. 2. _Flexurgebirge_--Flexure mountains. 3. _Horstgebirge_--Symmetrical block mountains. (b) _Faltungsgebirge_--Fold mountains. 1. _Homoomorphe Faltungsgebirge_--Homomorphic fold mountains. 2. _Heteromorphe Faltungsgebirge_--Heteromorphic fold mountains. II. _Rumpfgebirge oder Abrasionsgebirge_--Trunk or abraded mountains. III. _Ausbruchsgebirge_--Eruptive mountains.