In "We Wear the Mask," what does Dunbar say is shown on the masks worn by African Americans?

We wear the mask that grins and lies, it hides our cheeks and shades our eyes, This debt we pay to human guile; With torn and bleeding hearts we smile, And mouth with myriad subtleties. Why should the world be over-wise, In counting all our tears and sighs? Nay, let them only see us, while We wear the mask. We smile, but, O great Christ, our cries To thee from tortured souls arise. We sing, but oh the clay is vile Beneath our feet, and long the mile; But let the world dream otherwise, We wear the mask!