A classification scheme that grouped plutonic and metamorphic rocks together, but classified sedimentary rocks as volcanic would be based on a misunderstanding of the origins of these rocks.

- Plutonic rocks are formed from the cooling of magma deep beneath the Earth's surface.

- Metamorphic rocks are formed from the alteration of existing rocks due to heat, pressure, or chemical reactions.

- Sedimentary rocks are formed from the accumulation and cementation of sediments, such as sand, mud, and gravel.

- Volcanic rocks are formed from the cooling of lava on the Earth's surface.

Therefore, plutonic and metamorphic rocks are both formed from the solidification of molten or partly molten rock material, while sedimentary rocks are formed from the accumulation and cementation of sediments, and volcanic rocks are formed from the cooling of lava on the Earth's surface.