Preheat the oven to 375 degrees F (190 degrees C).
Pour the melted margarine into the bottom of a 9x13 inch baking dish; set aside. In a medium bowl, combine the flour, baking powder and white sugar. Stir in 1 cup of the reserved liquid from the peaches and the milk until smooth. Pour the batter evenly into the bottom of the dish over the margarine. Do not stir. Spoon peaches over the batter.
Bake for 35 to 40 minutes in the preheated oven, or until the top is golden.
