Krycek, I'd be happy to "guess"... But as that's probably all it would be, I don't know how much help I'll be. Lol!
Post that pic, friend! Let's have a look. :-).
Maybe it'll look like mine, someone else will answer, then we'll both know what we have! (See? Always the "ulterior motive" with me, lol!).

If it's not important to you don't bother.

Well, of course it's important to me mac. Otherwise I wouldn't have started the post! Don't gotta be so cranky!

Here's the pics.

This is in the family room, it's fully finished:

This is a close-up of an unfinished plank. The varnish peeled off. It's quite dark/red:

Family room again:

Closer up of the family room floor:

The whole main floor has the same varnish it's just we have much more light in the family room shining off the wood.

Sorry about the kitties, they insisted on making their presence known!

red oak

Is that common? Most of the hardwood in the other houses I've been in must have been white oak. They were oak but definitely not red oak. That's probably why I didn't recognize this one. Thank you!

With the grain being so fine/close/tight, I'd think it was white oak rather than red. The general age would also suggest white; red oak as flooring was unheard of 100 years ago, being a stinky and inferior wood compared to white oak. The dark shellac or other varnish (and the camera white balance) is affecting the color perception I think.
Casey

I agree that if you can be sure the floor was installed 100 years ago it is more likely to be white oak but all I know is what I see in the low resolution photos and I can't see the longer rays characteristic of white oak however they may be hidden by the dark finish.

If you can pull a piece up, post a photo of the end grain.

Would you treat if differently if it were white instead of red?

Since the photos are not very helpful it is the general uniformity of the wood that most suggests red oak to me.

Here are examples of unfinished white (above) and red (below). Perhaps you can see your flooring well enough to find the ray lengths.

Guessing about what would have been used is interesting but unhelpful; it is whatever it is.

Well it's just part of natural human curiosity, that's all...I'm a naturally curious person!

What has made the wood turn such a deep red? Is it time? Over time does it get redder? I know it's not stain that's dyed it red because I looked in the basement on the bottom of the hardwood and it's just as deep a red.

If you can see it from the basement and it's very red it might not be a hardwood but old growth Douglas fir.

btw, the main reason I asked, mac, was because we found out the second floor of the house has the same type of hardwood. If it was something valuable or unique, we would refinish it. If it was something mundane, we were considering floating something else over it.

The main reason I was curious because I've seen a lot of hardwood but never one this deep of a red so I was curious as to what it was. At first I thought it could have been a type of mahogany.

You know, with that color and fine grain, it could also be chestnut. It it is, you have something very fine and rare.
If you strip some of the finish and the color stays the same, then it becomes ever more likely. The only real way to determine for sure is to post a very clear close-up of the end grain of a piece.
Casey