I am retelling the exact story Yousef Awyan told me in the Cairo Museum about the Schist Disk!! I'd never heard of this backstory about it's possible use before!! If it turns out to be correct then ...

At the very bottom of the canyon lies the Vishnu Schist, a hard rock originally deposited mainly as sediments some 2 billion years ago. The layer was subsequently covered. Around 1.7 billion years ...

Is it likely that rock similar to the Vishnu Schist exist elsewhere but are not exposed at Earths surface? Explain ...