  Keeping in mind the possibility of large theta(13), which will be soon
explored by reactor and accelerator experiments, I formulate a perturbation
theory of neutrino oscillation under the ansatz, sin^2(theta_{13}) \simeq Delta
m^2_{21}/\Delta m^2_{31} \simeq 0.03, which is comparable to the Chooz limit.
Under the framework, I derive the perturbative formula of the nu_{e} appearance
probability valid to order \epsilon^2 in which effects of arbitrary matter
density profile is taken into account. I use the formula to analyze problem of
possible obstruction to detecting lepton CP violation by effects of asymmetry
in matter density profile. Though the asymmetry could be large for neutrino
trajectories which traverse both continental and sea crust, its effect on
obscuring CP violation measurement is found to be quite small.
