Hornfels is a metamorphic rock formed by the contact between mudstone / shale, or other clay-rich rock, and a hot igneous body, and represents a heat-altered equivalent of the original rock. This process is termed contact metamorphism. Because pressure is not a factor in the formation of hornfels, it lacks the foliation seen in many metamorphic rocks formed under high pressure and temperature regimes. Pre-existing bedding and structure of the parent rock is generally destroyed during the formation of hornfels. It is often difficult to identify hornfels without microscopic observation, or knowledge of its association with a magma body, as it is typically non-descript inhand specimen. Under a microscope the structure of hornfels is very distinctive, with small, generally equigranular, mineral grains fitting closely together like the fragments of a mosaic or a rough pavement.