What determines the difference between a slate and a gneiss rock that both are formed from a shale? What role does the parent rock play in determining the type of metamorphic rock that will be formed?

What determines the difference between a slate and a gneiss rock that both are formed from a shale? What role does the parent rock play in determining the type of metamorphic rock that will be formed? In a rock that had undergone contact metamorphism, would you expect to find foliation? Why or why not?