What is this old wall made of?

I just bought a fixer-upper. I started tearing off the wallpaper in a bedroom and noticed that the walls beneath have a few small holes (maybe the size of a quarter) and are crumbly. There is one spot where the wall is weak. I don't know what the walls are made of. I would assume plaster, but that just doesn't seem right. The material is maybe 1/2 inch thick and a brown color. It looks like an ancestor of drywall...but there is no white stuff on the inside. The house was built in 1908.

No comments about buying a fixer-upper when I obviously have very little knowledge - this house was *insanely* cheap because of the current market in my area. ;)