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a dense, fine-grained rock produced by the action of an instrusive body on the enclosing rocks. Hornfels differs in composition from the rocks that have been metamorphosed. Clay rocks become hornfels containing cordierite, andalusite, sillimanite, garnet, biotite, and feldspars. Basalts, basalt tuffs, and graywackes become hornfels with pyroxenes, basic plagioclase, and hornblende. Marls become hornfels with wollastonite, olivine, spinel, and carbonates. As the quantity of carbonates increases, hornfels become contact marbles.