The line “"He [Jack] began to dance and his laughter became a bloodthirsty snarling. He capered toward bill, and the mask was a thing on its own, behind which jack hid, liberated from shame and self-consciousness." is found in chapter 4, page 58.
This chapter displays an image of Jack slowly leaning towards barbaric behaviour and savagery. The painted mask does not just represent Jacks’ liberation from the rules set by Ralph, but also his acceptance of his own sadistic behaviour over the rest of the group.
