- Different classes of rocks are formed by different methods
- Class is a measure of how a rock is formed
- Therefore, the method by which a rock is formed can be used to classify rocks

- The method by which a rock is formed can be used to classify rocks
- The origin of a rock means the method by which a rock is formed
- Therefore, the origin of rocks can be used to classify rocks

So, the origin of rocks can be used to classify rocks
Question
Rocks can be classified many different ways, including as igneous, metamorphic, and sedimentary. This classification is based on the

Context
- Different classes of rocks are formed by different methods 
- Class is a measure of how a rock is formed 
- The origin of a rock means the method by which a rock is formed