I believe this is not rock, it appears to actually be pieces of street asphalt mixed with tar. I used to lay streets with my uncle on occasion and this looks very similar to what we would pull up, before laying down the new roadway. It's an old mix they used to use for roads which had more tar than concrete, allowing a smoother ride, but making it last alot less, Those bubbly porous spots are where the tar was allowed to heat before cooling fully when mixed with the concrete. At least that's what it looks like to me. At Any Rate, that is one of the bigger chunks i've seen. Usually that stuff breaks up into small pieces.