In the third passage, Golding uses symbolism when he writes about Henry's footprints.  In the statement, "His footprints became bays in which they were trapped and gave him the illusion of mastery," the "they" referred to is the creatures in the sand, and when Henry stepped on them it gave him a strong sense of power.  This feeling of power was really important to Henry because it was something that he lacked within the community of boys on the island, so having control over the little powerless insects was more monumental in this story than if would have ordinarily been. By Henry experiencing this small amount of temporary control or "mastery", he is able to continue on and be somewhat content, even though he lacks control among others on the island.