Dissolve soda in boiling water. Add molasses and stir well. Set aside. Combine flour and sugar, and rub in butter to make crumbs. Have a 9-inch unbaked pastry shell. Pour in 1/3 of the molasses mixture. Alternate the layers having crumbs on top. Bake in 425 degree oven for 10 minutes. Then reduce to 350 degrees for 40 minutes.