The following quotes are all from chapter 3 of Golding's The Lord of the Flies.
His sandy hair, considerably longer than it had been when they dropped in, was lighter now; and his bare back was a mass of dark freckles and peeling sunburn.
This description of Jack creates an image of a boy who is in the process of succumbing to the natural elements that surround him. The relentless sun has bleached his hair and damaged his skin, and both of these effects of the strong sun represent the powerful effect of nature and the island environment on Jack. As well, the passage of time is observed in the growing length of his hair.
He tried to convey the compulsion to track down and kill that was swallowing him up.
"I went on. I thought, by myself—"
The madness came into his eyes again.
Jack's primitive instincts are beginning to take over his rational mind, as evidenced by his obsession with hunting and killing a pig for meat. In this passage, Jack is talking with Ralph, who continues to be able to maintain a firm grasp on his reason, unlike Jack. During this conversation, Ralph challenges Jack's primitive impulse to abandon all attempts at saving themselves in the pursuit of killing; though Ralph is unable to deter Jack, the reader is able to observe the growing chasm between the two boys and the expectations they have around the survival effort.
Evening was advancing toward the island; the sounds of the bright fantastic birds, the bee-sounds, even the crying of the gulls that were returning to their roosts among the square rocks, were fainter. The deep sea breaking miles away on the reef made an undertone less perceptible than the susurration of the blood.
In this passage, Golding employs vivid imagery to bring the island experience to life for the reader. The night time "advances" like a military force, and the sounds of the island are varied as daytime transitions to evening. The mention of blood in comparison to the sound of the sea is threatening, foreshadowing violence that already exists in Jack's imagination.