In chapter four, when Jack first paints the mask on his face, his character and personality morph into something much darker under the freedom of his new disguise.
"Beside the pool his sinewy body held up a mask that drew their eyes and appalled them. He began to dance and his laughter became a bloodthirsty snarling" (64).
The mask hides Jack away--not just from nature, but from the other boys. Wearing the mask frees Jack to become a much more ruthless version of himself. Where Jack formerly balked at the idea of bloodshed and hesitated killing the piglet, the masked Jack feels no such restrictions--as if he has freed himself from the boundaries and restraints of civilization.
"He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness" (64).
This moment in the novel when Jack first begins to wear the mask signals his transition from modest choir boy to ruthless savage.
Preferably *nature* quotes related to change in characters