Sub-floor in 1950's house

The bitumen is probably both a levelling compound and insulation in one. In my house the bitumen was laid after the skirting and kitchen were fitted. Where I have removed the hearth and kitchen units I used a self levelling compound mixed with sand to raise the missing bits to the existing level.

Adam

The bitumen might be a damp proof course.

Alternatively, it might have been a glue for a floor covering. Both 1950's lino tiles and wood block would have been stuck down this way.

The floor surface you refer to is Asphalt. Asphalt is a mixture of bitumin and fine gravel, heated in a tank pulled by a lorry. Its uses are for floors, flat roofs and tanking. When laid for flooring a layer of Batiscloth ( Batiscloth type of course felt with a bitumen mixed in) is put down first and the hot asphalt spread on top. When it get old it can be lifted easy, it comes up like a slab of cold toffee.