Fossils got it right... it is slag from iron-ore refining. The reason refinery slag gets found in so many places is, it's used as a much cheaper substitute for gravel. Refineries produce tons of it, and they practically beg you to come haul it away for them. So, for many hundreds of years it has gotten used as super-cheap-"aggregate" for railroad-track beds, and "gravel roads," and even sidewalks.
I can testify from my own metal-detecting experience that the sidewalks in Colonial-era Fredericksburg VA were made of slag. When that city decided to replace the modern concrete-slab sidewalks with brick walks in the downtown Colonial part of town, it created a brief window to metal-detect for coins. So many pre-20th-Century coins (and even civil war bullets & uniform buttons) turned up that I wrote a metal-detecting article titled "Under The Sidewalks of Fredericksburg" for the North/South Trader Civil War magazine (1983-ish, I think). But the problem was, the omni-present slag in the original Colonial sidewalks drove my detector crazy, due to the concentrated iron content remaining in the "bubble-y" slag.
