Prepare pastry. Preheat oven to 475 degrees F. Blend sugar, tapioca, and salt, then add cherries and let stand while you roll out the dough. Line bottom of 9" pie pan with one crust, fill with cherry mixture, sprinkle with cinnamon, then dot with butter. Cover with top crust and seal edges, cutting vents. Bake for 10 minutes, then reduce heat to 400 degrees F and bake a half hour, until crust is crisp and filling is bubbling. Cool and serve.