The Historic District

most likely bricks very similar to your outside walkway I would guess - they crumble with age and wear

Maybe so. Do you recall how mammoth the brick base for the fireplace is? I assumed that was solid brick, now I'm really wondering.
For anyone interested follow the link to my Instagram and scroll down. There's a picture of my daughter standing in front of it.
https://www.instagram.com/kettle_and_herb/

After talking with a historic architect I now understand that this is called a "saddlebag hearth." The brick enclosure consists of dirt and garbage, basically a midden, and then lime mortar was spread on top and then the hearth was laid on top of that. The architect postulated that the house might have been Yankee built because most Southern cooking fireplaces are on exterior walls, putting them on interior walls is a Northern thing to do. A local old house expert said that was generally true, but you do see internal fireplaces on inns and taverns which is what we suspect this house was. So, mystery mostly solved though I still think the more recent owners might have done something better than pouring concrete over it. I guess I should be thankful that they didn't tear the whole thing out.

My early 1800 house in NS has the same type of hearth. An angled timber crib underneath, holding rubble and lime mortar. Not very attractive, but fire-safe. It's a central chimney with 2 hearths; one cooking and another smaller, and another false one where there used to be a parlour stove.

That's interesting, Mal. We don't know yet if the dining room side also had a crane, or if that hearth is solid. Time will tell.
The upstairs fireplace in the log cabin addition has no hearth at all, the floorboards run right up to the opening. We have long suspected that the cabin was moved here from elsewhere and I guess that's the most logical conclusion to make about the missing hearth.