What does Peter do to Anthony Duncan in Chapter 17?

Peter hits Duncan in the head with a large rock in his bid to escape.

I stepped in silently, raised my arms above my head, and brought them down. There was no sound but that of the blow and the dry snapping of wood as he pitched forward across the hearth. I backed into the open air, feeling fainter and sicker than I had for a long time. A drop of rain splashed on my cheek. I shivered and pulled myself together with an effort.