It has been suggested that Ironstone Mountain is named after the sedimentary rock ironstone. This is plausible. However, there was a columnar basalt rock outcrop near the summit at 6400 ft up (see this picture
). That's pretty high in elevation for columnar basalt but not unheard of. So I checked the geologic map of Washington (WARNING: large PDF file to download so don't click this unless you have time to wait: http://ngmdb.usgs.gov/Info/dmt/docs/schuster07b.pdf
) and took an excerpt of the Ironstone Mountain locale. Where I have marked the summit of Ironstone is my best guess. This would put the summit in a Tertiary intrusive rock formation. But columnar basalt is an extrusive rock. So I don't know. I will say I've never heard of a sedimentary rock having a vertical, columnar orientation such as shown in the photo, so if it is ironstone that's interesting.