I had begun to wonder if an alien space ship had dropped her off in Haviland Hollow, New York because I could never find her on any passenger lists.
Then today I looked yet again, and I found on ancestry.com, a "Mary Hart" age 25 indexed as arriving on 17 June 1848 in New York. Since Mary had told the 1900 US Census that she arrived 52 years earlier, an arrival year of 1848 sure sounded right.
Here is the actual document:
and looking closely at passenger number 70: