I lifted a friend's old rigid stumpjumper, it was suprisingly light. Easily under 25 pounds. With some modern parts it would make a good steel cross bike. Stumpjumpers, as opposed to hardrocks, have always been pretty light with racey geometry. Rockhoppers are somehwere in between, probably depends on the year.

I got my butt handed to me last race by a guy on an old rigid mountain bike (was high end in its day, actually) during my last race.