This is a Big-leaf aster, Aster macrophyllus.
As a first cut, a lavender aster on Isle Royale is probably a Big-leaf aster, while a white aster is probably a flat-top aster. The flat-top aster also has longer, slender toothless leaves, while the big-leaf has leaves that are bigger (duh) and toothed.
This was photographed on Scoville Point near Tobin Harbor, August 2007.