The first stump - the one with the wide white cambium (sapwood) and multi-colored center - could be walnut, I guess, but it looks an odd color and overall seems to be a little atypicall, due to the wide, uneven cambium layer and multi-colored wood. I don't know what type of tree it is. The shattered wood looks like walnut, but it is hard to tell what the grain looked like before it shattered. The next stump - the dark-centered one - looks like typical Black Walnut. The last shot of a log the bark looks like it could be Black Walnut, but I can't see the wood color in the picture. Wood color would cinch the ID. Black Walnut is a very dark wood, darker than other common woods of the eastern US. I don't think the slab in the second post looks like Black Walnut, at least not on my screen. It looks way too light in color.
Black Walnut is valuable wood if you get a nice, big straight tree, but around here there are tons of smaller, crooked Walnuts that end up as firewood. not every walnut is veneer quality. It burns nicely but starts to burn a little more slowly than cherry or soft maple, which are similar in BTUs.