Roger is throwing rocks at Henry is Chapter 4.
When Roger is throwing rocks at Henry, he is afraid to throw rocks directly at him. Instead, he throws the rocks near him.
Roger stooped, picked up a stone, aimed, and threw it at Henry— threw it to miss. The stone, that token of preposterous time, bounced five yards to Henry’s right and fell in the water. Roger gathered a handful of stones and began to throw them. (Ch. 4)
The reason that Roger is afraid to throw the rocks at Henry is because he is still held back by the cultural norms of society. It is wrong to throw rocks. This is something he has always been told. When he sees the child and throws rocks at him, he is reminded of this.
Yet there was a space round Henry, perhaps six yards in diameter, into which he dare not throw. Here, invisible yet strong, was the taboo of the old life. Round the squatting child was the protection of parents and school and policemen and the law. Roger’s arm was conditioned by a civilization that knew nothing of him and was in ruins. (Ch. 4)
Henry is frightened at first, but then laughs, and looks for “the friend who was teasing him” (Ch. 4). He then gets bored with this and walks away. Jacks calls Roger away, and they paint themselves in order to hunt the pig. In doing this, though, what they are really doing is passing over into the savage world from the civilized one.
When Roger throws the rocks, he is on the threshold of savagery but does not cross over. Something holds him back. Jack, however, recognizes in Roger the potential for true savagery and awakens it. When the boys paint their faces and do the pig chant and dance, they become savage and leave the civilized world behind by degrees.