These are sometimes and archeology project to understand what was done to get it to the state it is now. Also, you should put your location in your profile - it can make a difference in what advice you're likely to get.
It isn't uncommon for a small farm building to have been built on a single layer stone foundation, even where the ground freezes a few feet down, or it may have been several courses of stone. They can be post and beam, pole, balloon, or conventional framed. The concrete floor may be original, or it may have been added later. You get the idea.
First, get a shovel and dig next to the rock foundation to see if it was back filled to raise the level up high as it is, and see roughly how far down the stone foundation goes. A later grade change is more common than building these so the sills are below grade. It's possible that it has settled several inches over the years, resulting in the sill being below grade, but thee wouldn't have been much slab settling. Also, uncover the sill on the outside so you can see how bad it is - it could be dust by now. You also should check to see if there has been significant settling relative to the concrete slab - not always possible to tell conclusively.
In a case like this, you may wind up using a ground contact rated PT beam with a mud sill on top to replace what was there, and do a lot of stud trimming. masonry and concrete doesn't handle movement of old stone foundations well.
I've spent the last 4 years in construction. But I have very little experience in buildings that are 100+ years old like my house. So I've got a little bit of a learning curve here.