Metamorphic layer

Either I don't understand the metamorphic process or the process is different than explained.  How does a layer of metamorphic rock exist between two layers of sedimentary deposits?

I suppose that the layer is lime stone which is a precepitate while the upper and lower sedimentary layers are erosion deposits. Which could mean that the lime was deposited while the world was dry or cold and the ocean was very stable and possibly very deep resulting in the hard lens.