CHOCOLATE MARBLE CHEESECAKE

1-1/2 cups graham cracker crumbs
1/3 cup melted butter
1 tablespoon sugar
3 packages (8 oz each) cream cheese
1 cup sugar
1/4 cup flour
1 teaspoon grated orange peel
8 ounces sour cream
1/2 teaspoon cream of tartar
6 ounces semisweet chocolate, melted and cooled
6 egg yolks
2 tablespoons orange liqueur
1/2 teaspoon salt
6 egg whites
1/4 cup sugar

Combine cracker crumbs, 1 tablespoon sugar, and butter in small bowl. Press evenly on bottom and sides of 9-inch spring form pan. Soften cheese in large bowl. Add 1 cup sugar, gradually;
beat until smooth. Add yolks, one at a time, beating well. Stir in flour, liqueur, peel, and salt. Blend in sour cream. Beat egg whites and tartar until foamy. Add 1/4 cup sugar gradually. Beat until stiff.
Heat oven to 300o F. Fold whites carefully into cheese mixture. Measure 3 cups mixture into small bowl; blend in chocolate. Drop alternate spoonfuls of cheese mixture and chocolate mixture into pan, reserving about 1 cup cheese mixture and 1/4 cup chocolate mixture. With metal spatula, cut through mixture in pan to marbleize cake. Spread reserved cheese mixture evenly on top. Make parallel 1-inch lines on surface with chocolate mixture. Run tip of knife at right angles to lines and in opposite directions for a feathered effect. Bake 1 hour. Turn off oven; let cake stand in oven 1 more hour. Remove and cool on rack. When cool, remove from pan and chill.