The surface of a topological insulator is a closed two dimensional manifold. The surface states are described by the Dirac Hamiltonian in curved two dimensional spaces. For a slab-like sample with a magnetic field perpendicular to its top and bottom surfaces, there are chiral states delocalized on the four side faces. These ``chiral sheets'' carry both charge and spin currents. In strong magnetic fields the quantized charge Hall effect ($\s_{xy}=(2n+1)e^2/h$) will coexist with spin Hall effect.