What was the thin rock layer called that the characters fell through in Journey to the Center of the Earth?

Near the beginning the movie, Hannah, Trevor, and Sean find their way to a cave with deposits of diamond, emerald, ruby, and feldspar. Quickly they discover that they are actually standing on a very thin layer of rock and end up falling through that rock layer.


I forget the name of this rock layer, but it would be SO helpful if someone could remind me.


Thanks bunches!What was the thin rock layer called that the characters fell through in Journey to the Center of the Earth?
I think its muscovite





Hope this helps

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