Here's why:

* Marble: Formed from the metamorphism of limestone.

* Gneiss: Formed from the metamorphism of granite or other igneous rocks.

* Slate: Formed from the metamorphism of shale.

* Shale: A sedimentary rock, not a metamorphic rock.

Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical reactions. Shale is formed by the accumulation and compaction of sediment.