Spread half of the sweet potatoes in the bottom of a greased 13x9 dish.  Top with half of apples and half of marshmallows.  Repeat layers.  Combine sugar, salt, and cinnamon.  Sprinkle over casserole.  Dissolve cornstarch in water; pour over sugar mixture.  Dot with butter slices; cover.  Bake at 350 for 1 hour.