E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The era between the death of Charlemagne and the close of the Carlovingian dynasty is so called from its almost ceaseless wars. It is sometimes called the leaden age for its worthlessness, and the dark age for its barrenness of learned men.
Iron Age. The age of cruelty and hard-heartedness. When Hubert tells Prince Arthur he must burn his eyes out, the young prince replies, Ah, none but in this iron age would do it. (Shakespeare: King John, iv. 1.)