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I found an Iowa 1885 census with Elizabeth Stewart in Mt. Pleasant, age 64, widowed, living with John Houston, age 23, and Mary J Houston, age 20. The censuses I have found (the 1885 one says born 1821, and the only other one I found is the 1900 one, which says born Oct. 1821, so I would assume that is correct). So marriage to Higham must have occurred after 1885.
I don't live in Henry County so I don't have any access to the cemeteries. Most that I have been to are just graves, no other buildings, except maybe within Mt. Pleasant proper. Any information I've gotten has been from the genealogy section of the public library. Wish I could help more.