In chapter six, "Beast from the Air," Ralph becomes very angry when the boys reach Castle Rock and he sees that the mountain does not have a signal fire:

"He clenched his fist and beat hammer-wise on the red wall at his right.  His lips were tightly compressed and his eyes yearned beneath the fringe of his hair" (107).

Ralph hits the rock as a way to diffuse his anger at the situation; he is supposed to be the leader, but feels frustrated that everything seems beyond his control.  Only moments later Ralph instructs the boys to move on toward the mountain; in the face of the hunters' open disagreement with his orders, Ralph punches the rock again and "struck the skin off his knuckles.  They did not seem to hurt" (108).  Ralph does not seem to mind his bruised knuckles, because his emotions are all tied up with the boys' refusal to follow his directions.  He feels incredibly frustrated by their lack of motivation to maintain the signal fire on the mountain to be rescued, and their childish behavior and antics have made him furious.  Much later, he will probably regret having skinned his knuckles like that, but at the time being, all he can focus on is Jack and the hunters' mutinous actions.