For, about twelve feet away from me, John and Mary Cavendish were standing facing each other, and they were evidently quarrelling. 
I happened to walk in on John and Mary Cavendish who were arguing with each other. 
mnli hypothesis: I happened to walk in on John and Mary Cavendish who were arguing with each other.  premise: For, about twelve feet away from me, John and Mary Cavendish were standing facing each other, and they were evidently quarrelling. 