Terrigenous Rocks(T) examples: mudrocks, sandstones, and conglomerates. Impure Allochemical Rocks(IA) examples: very fossiliferous shales; sandy fossiliferous or oolitic limestones. Impure orthochemical Rocks(lo) examples: clayey microcrystalline limestones. Allochemical Rocks(A) examples: fossiliferous, oolitic limestones and dolomites. orthochemical Rocks(o) examples: limestone, dolomite, anhydrite, and chert.