if something can be used to determine a property of something else then that something can likely be used to determine that property of a part of that something else & age is a property of rocks ->  if something can be used to determine the age of a rock then that something can likely be used to determine the age of a part of a rock  if something can be used to determine the age of a rock then that something can likely be used to determine the age of a part of a rock & the law of superposition can be used to determine the relative age of the rock ->  the law of superposition can be used to determine the relative age of part of a rock  the law of superposition can be used to determine the relative age of part of a rock & sedimentary rock is a kind of rock ->  the law of superposition can be used to determine the relative age of a part of sedimentary rock  the law of superposition can be used to determine the relative age of a part of sedimentary rock & layers are a part of sedimentary rock -> the law of superposition can be used to determine the age of sedimentary rock layers