Hello,
I have no connection to the CHISHOLM family, but I can tell you [since I live in Danville, which is an "independent city" within the borders of Pittsylvania County] that Pittsylvania County is the largest county in Virginia and is located on the VA-NC border, due north of Greensboro, North Carolina.
If you find Greensboro on a map and follow the path of Route 29 north into Virginia, you'll find Danville and thus Pittsylvania County.
Hope this helps!
Richard