What is this surface?

I have just lifted carpet from this floor in preparation for laying ceramic tiles. Although this part of the house is over 200 years old I believe the base here was probably put down in 1976. There is a fairly hard, smooth, dark coloured surface on top of an unscreeded base. The surface is 1 - 2mm thick and looks as if it's had some adhesive applied to it in the past. Much of it is sound but in some areas, as pictured here, the surface is detached from the base so I plan to remove it, seal the base and apply a levellling compound. What is the surface and will it be ok to tile over the sound areas?

It maybe the floor screed ratio mix was a bit weak at the time (sandy ?) so they put a thinner stronger cement mixed with soft sand over it or it could be a levelling compound. Might be best to try and remove as much as you can then seal it with floor sealer before with a fresh flooring levelling compound.
Is the floor damp ?

Thanks for the instant response! No the floor shows no sign of dampness.