Hi - thanks for writing a response. But how are you relating those comments to this thread? And who is this great-grandfather who is popping into this thread at such a late date? Is he someone we discussed?
Seems you have this question: "Is my great-grandfather the father of one or more children outside his marriage? If so, are any of his children the offspring of a Native American mother?"
To get to the bottom of your question, you'll want to name a specific great-grandfather. (if you know which great-grandfather it was). After identifying him, you would then seek the birth certificates of each of his known children and seek their names on Indian census rolls and seek their ethnic identity on U.S. census rolls and so on. Could be a very difficult task to track down illegitimate children, and you'd want to add as many clues as possible from your family history and family stories (beginning with the name of the guy).
And yes I've known Ojibwe people with the surname Defoe. That was not the issue. The issue was - why are you bringing them up?