Hi; I have a very comprehensive family tree including this John & Mary that dates back to 1740 (have hit a block there at this point). Would be more than happy to share it with you if you haven't managed to get the info yet :)
To answer your question, however, John William Shearer (m. Mary Drever 1870) died in the Orkney famin (I think) & Mary moved down to Sunderland with her children to live with her brother - a yeast salesman I think. Their decendents still live there now (he was my grandfather's grandfather).
As I said, let me know if I can be of any more help. :)