  We develop a vertex formalism for topological string amplitudes on ruled
surfaces with an arbitrary number of reducible fibers embedded in a Calabi-Yau
threefold. Our construction is based on large N duality and localization with
respect to a degenerate torus action. We also discuss potential generalizations
of our formalism to a broader class of Calabi-Yau threefolds using the same
underlying principles.
