Rocks may differ in many ways. For example: Composition- The mineral composition is one way that rocks can differ. Some rocks, described as felsic, contain more silica and less iron and magnesium than mafic rocks. Rocks can further be divided by the proportions of these and other elements that they contain, they can differ by the composition of the rocks from which they are formed. Limestone can be formed as a biochemical sedimentary rock (fossiliferous limestone), a chemical sedimentary rock (travertine), or a clastic sedimetary rock (clastic limestone). Metamorphic rock is formed when a rock undergoes pressure and/or heat without melting. The amount of heat or pressure as well as the direction of any applied pressure influences the type of metamorphic rock produced. The amount of time over which the rock is exposed to pressure or heat also affects the rock produced.