Preheat the oven to 425°F. Line a pie pan with half the pastry dough. Drain the cherries, saving 1/2 cup of the juice. (If you are using fresh cherries, the natural juices which bubble up during baking are sufficient.) Mix the juice, sugar, and tapioca in a bowl, add the cherries, and toss well. Pile into the lined pan and dot with the butter. Roll out the top crust and drape it over the pie. Crimp the edges and cut several vents in the top. Bake for 10 minutes, reduce the heat to 350°F, continue to bake for 30- 40 minutes more, until the crust is lightly browned.