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