classifying is when one sorts something by type & a class of something means a type of something ->  classifying is when one sorts something by class  classifying is when one sorts something by class & class is a measure of how a rock is formed ->  rocks can be classified by how they are formed  rocks can be classified by how they are formed & class is a property of a rock and includes values of sedimentary / metamorphic / igneous -> rocks are classified as igneous, sedimentary, or metamorphic based on how they are formed