Soak cherries in water for 10 minutes. Drain and set aside. In a large mixing bow, combine the flour, sugar, baking powder, salt and cream of tartar. With a pastry blender, cut in the margarine. Set aside. In a small bowl, combine egg yoke, sour cream, cream and extract. Add to flour mixture; stir until a soft dough forms. Turn out onto a lightly floured surface; knead gently six to eight times. Knead in cherries. Divide dough in half and shape into balls. Roll each ball into a 6 in. circle. Cut into six wedges. Repeat with remaining ball. Place on lightly greased baking sheet. Beat the egg white until foamy; brush tops of scones and sprinkle with sugar. Bake at 400 degrees for 15-20 minutes. Serve warm.