What is this aggregate under my soil

[Nishing]

Hello, i am doing a small patio and started removing the top soil from the area and i found lots of bricks with this black granular stuff inbetween them, i would like to know what it is and whether i need to remove it.

Its like black perlite, its easily crushed into powder inbetween my fingers and it leaves black smudges. I thought it must be coal slag but i looked for pictures of coal slag and im not sure if its the same stuff.

Anyone know what it is?

Thanks

[lutonlagerlout]

sounds like fly ash,they used to use it as subbase
cant see it so cant say if its any good but there loads buried in my garden
LLL

[Nishing]

Yes i think you may be right, there used to be tarmac in the area i guess that would make sense.

Thanks