Pretty cool. Can't say I've ever ran across a house or building like the OP posted.
Looks like rough cut lumber or as was done in the old days, a true 2x4. Wonder if it's something like chestnut or locust? Can't imagine most other woods would hold up untreated. Surely there is others, but those 2 hold up to the elements and chestnut would've been around yet in all likely hood when that old place was constructed.
The OP said it's in NE Oregon. Most of that country is pretty dry. As long as it's sitting on rocks or cement to avoid contact with the soil, pine or fir can last a long time. I've seen many abandoned shacks here in so. Idaho where it's dry and some have the wood in pretty good shape for being 100+.
