Answering my own question -
Mary Nicol/Bell/Beattie went to U.S. In their 1880 census she was 60 in Pennsylvania living with one of her Beattie sons, James 19 now spelt Beatty, a coal miner.
The other Beattie son Thomas 22 now spelt Beaty, also a miner, was in the same state, married in US to Ella 19, and they have a baby daughter Mary 0.
When they emigrated I can't find, but if Lodger is correct about old Thomas Beattie having died in 1879, they ( including Thomas ! ) appear to have moved pretty fast.
Already in Pennsylvania was one of Mary's Bell children, Margaret, who had married Robert McDill ( Middle/Medill ) before she emigrated in 1866 along with their young family.
Also in that party was her brother Duncan Bell who had been born after his father Duncan was killed in the Wishaw mine. And Duncan jnr became ......a miner.