I know of a Christian Farfar (Farquhar?) who married an Alexander Keith. He was born 1681 in Kincardineshire, Scotland, and died on July 2, 1721 in Baltimore, Maryland. I noticed that your message said they were married about 1807--could that be 1707?
They had a son, John Keith, who in turn had a son named Alexander. My lineage descends through them.
I hope this helps, but I admit, that my sources are not proven yet.
If you have come upon more information than this, I'd love to hear about it.