There is some data on Presbyterianism in Wash. Co. at Citizens Library. The Presbyterian church has a Historical Society located in Philadelphia. I recently returned from Chester Co., PA History Center & they have much on the Presbyterian Church's beginnings in that area. The Rev. John MacMillan is mentioned in these publications as getting his education at "Faggs Manor" plus more. Reading about his establishing the Presbyterian Church west of the Alleghenies would be a good place to start.