The fundamental form-elements may be reduced to the six proposed by Professor Penck as the basis of his double system of classification by form and origin.[32] These may be looked upon as being all derived by various modifications or arrangements of the single form-unit, the _slope_ or inclined plane surface. No one form occurs alone, but always grouped together with others in various ways to make up districts, regions and lands of distinctive characters. The form-elements are: 1. The _plain_ or gently inclined uniform surface. 2. The _scarp_ or steeply inclined slope; this is necessarily of small extent except in the direction of its length. 3. The _valley_, composed of two lateral parallel slopes inclined towards a narrow strip of plain at a lower level which itself slopes downwards in the direction of its length. Many varieties of this fundamental form may be distinguished. 4. The _mount_, composed of a surface falling away on every side from a particular place. This place may either be a point, as in a volcanic cone, or a line, as in a mountain range or ridge of hills. 5. The _hollow_ or form produced by a land surface sloping inwards from all sides to a particular lowest place, the converse of a mount. 6. The _cavern_ or space entirely surrounded by a land surface. Geology and land forms. These forms never occur scattered haphazard over a region, but always in an orderly subordination depending on their mode of origin. The dominant forms result from crustal movements, the subsidiary from secondary reactions during the action of the primitive forms on mobile distributions. The geological structure and the mineral composition of the rocks are often the chief causes determining the character of the land forms of a region. Thus the scenery of a limestone country depends on the solubility and permeability of the rocks, leading to the typical Karst-formations of caverns, swallow-holes and underground stream courses, with the contingent phenomena of dry valleys and natural bridges. A sandy beach or desert owes its character to the mobility of its constituent sand-grains, which are readily drifted and piled up in the form of dunes. A region where volcanic activity has led to the embedding of dykes or bosses of hard rock amongst softer strata produces a plain broken by abrupt and isolated eminences.[33] Classification of mountains.