The only explanation in this situation is that the parent rocks of the marble and slate were different. This indicates that the region once held both shale and limestone before metamorphism.
In reference to rocks, it denotes those that have undergone metamorphism—rocks that may have started as sedimentary, igneous, or another type of metamorphic rock but has since undergone "recrystallization" and taken on an entirely new form—under new temperature and pressure conditions.
For instance, when limestone is heated and compressed, it changes into marble, a more coarsely crystalline and occasionally banded rock.
When shale, a clay-rich, soft rock, is compressed, slate, a tougher rock, is created.
The alterations are frequently even more remarkable, resulting in the formation of new minerals, including garnet, mica, and tourmaline (as seen in schist).
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