People are thinking that this is not Alexander. But there appears to be a big gap from 1870 to 1877?
Not necessarily a gap.
It is just that the last
recorded baptism for that couple was in 1870.
It is quite possible there were other children, who like Alexander, do not have birth / baptism records.
Unless Marjory died early, or was quite old when she married, one would normally expect a woman to have children across a 15-20 year period. So it is possible that during the 1770s there were another 4 children (including Alexander), whose records no longer exist.
Having a look at the 1841 census for Argyllshire, there are quite a number of very elderly people with the Graham surname that could be from the same family line.