What does it mean to be placid? (From Charlotte Bronte's Jane Eyre)

Undisturbed; tranquil; calm; quiet.

While St. John's blue eyes and ivory skin align him with ice, Rochester's dark hair and passionate nature connect him with fire. Jane immediately detects a restlessness or hardness under St. John's seemingly placid face, however.

From Charlotte Bronte's Jane Eyre.