I figured "chat" was gravel or something from the context, but had never heard the term before. Maybe I'm the lone idiot, but in case anyone else is confused:
"Chat is a term for fragments of siliceous rock, limestone, and dolomite waste rejected in the lead-zinc milling operations that accompanied lead-zinc mining in the first half of the 20th century. [...] Although poisonous, chat can be used to improve traction on snow-covered roads; as gravel; and as construction aggregate, principally for railroad ballast, highway construction, and concrete production."
https://en.wikipedia.org/wiki/Chat_%28mining%29
