A sleeve-worn stone of casement?

A sleeve-worn stone of casement?
You must be reading MacLeish's "Ars Poetica." I take this line to mean that the stone casement ledge of the window is worn from people leaning on it. Although this sounds unlikely, some stones such as marble are soft. You may have noticed, for instance, that marble stairs wear down in the middle after many feet have traversed them.Apparently, the building described here is quite old. This line aways makes me think of the "lonely men in shirtsleeves, leaning out of windows" in Eliot's "Love Song of J. Alfred Prufrock." However, MacLeish wants us to think about a vacant or ruined place that still bears the imprint of human activity.