Charles Dickens symbolically entitles Chapter XXIV of Book the Second "Drawn to the Loadstone Rock"; the loadstone, a variety of magnetite possessing a magnetic polarity, symbolizes the pull of France to Darnay in his guilt over the sins of his father and, especially, his uncle. Added to this guilt, Darnay cannot allow Gabelle to suffer for the injustices of his family.
Like the mariner in the old story, the winds and streams had driven him within the influence of the Loadstone Rock, and it was drawing him to itself, and he must go.
His personal sacrifice of returning to France in response to the written pleas of Gabelle to show his expiation for the mistreatment of the peasants at the hands of the Evremonde family, of course, serves to further the narrative and provide Darnay's doppelganger, Sydney Carton, his opportunity for redemption, as well. But, Carton's main purpose is to "stay bloodshed," and to "assert the claims of mercy and humanity."