"And as the smart ship grew/ In stature, grace, and hue,/ In shadowy silent distance grew the Iceberg too." Markus's daring gambit in "Across an Untried Sea" is that Charlotte Cushman's counterpart in Britain (her iceberg, so to speak) was Jane Welsh Carlyle, a surgeon's daughter from outside Edinburgh who submerged her considerable talents in the service of her husband, the Victorian sage Thomas Carlyle. Markus takes her cue from Geraldine Jewsbury, a popular novelist who knew both women and portrayed them as "half sisters" in a novel of that title. Alien they seemed to be —so alien, in fact, that Carlyle ("the most caustic, the most concrete, the most clear-sighted of women," as Virginia Woolf described her) shied away from meeting Cushman. But eventually, in Cushman's triumphant words, "She came —she saw and I conquered."

Rule, Cushmania! What must she have been like on stage? A force, a fury, a revelation. Photographs by Matthew Brady and the Boston team of Southworth and Hawes can barely contain her imperious face and jagged form. During the Civil War, she played Lady Macbeth, her signature role, opposite Edwin Booth. A week later, she teamed up with Booth's brother, John Wilkes, a madman she thought, even then, before his most famous traversal of the stage. When Cushman needed money for her "belongings" (as she called her entourage of friends and female lovers and her free black servant, Sallie Mercer), she went on tour. "Farewell tours," a specialty of hers, paid best. During the final yeas of her life, dying of breast cancer, Cushman toured relentlessly: first acting, then standing still, then sitting at a desk as the pain became unbearable. At her farewell from the stage in New York in 1874, 25,000 fans gathered to pay her tribute.

Offstage, Cushman was no less a force. With her weapons collection and her pleasure in the company of powerful men —she knew and admired Lincoln and schemed with his secretary of state, William Seward —she seems like an Ibsen heroine: Hedda Gabler, say. Generous and needy, she adopted her daft nephew, Ned, then induced her own lover, Emma Crow, to marry him. In its cruelly cunning geometry, it was an arrangement worthy of Henry James, and may even, Markus suggests, have inspired the plot of "The Golden Bowl."

Jane Carlyle, by contrast, lived out her life in sickrooms and darkened parlors. The parched territory of the Carlyle marriage has been shrewdly surveyed before —notably by Elizabeth Hardwick and Phyllis Rose. Eager to enlist Carlyle in the ranks of what the Victorians called "independent women," Markus accepts a little too easily the rumors that Carlyle's husband was impotent, that she died a virgin. (Markus notes, as if it clinches the argument, that Carlyle didn't know the sex of her own cat.) Though she doesn't quite argue that Carlyle was a lesbian, she does push the notion that "Jane's female friendships had taken on a highly charged quality."

"Dared and Done" was the title of Julia Markus's previous biographical venture, in which she broke the shocking news that the Brownings —the most famous lovers of their time —avoided having children for fear that a racial taint on Elizabeth Barrett's side might reveal itself in their offspring. No comparable bombshell explodes in "Across an Untried Sea." There is no climax, no great revelation. Even the meeting of the two "half-sisters" is muted, hardly Hardy's titanic "consummation" that "jars two hemispheres." The considerable power of the book lies in part in the sustained local verve of the writing: Henry James "alluded to the sexuality of 'our set' in Florence in tones so hushed in shadowy illusion that he could as well be referring to lichen on a villa wall."

The ingenious structure of Markus's book is even more striking. Markus is the rare kind of biographer who sees a human being as a web of relationships rather than a hard kernel of selfhood traversing time and space. Relentless networkers both, Cushman and Carlyle lend themselves to such treatment. "The great emphatic range" of Cushman's acting, Markus observes, "had something to do with the centrality of other people to her own well-being. She seemed to find her own reality through others." One of the great letter-writers of her time, Carlyle spun her own human web through her extraordinary correspondence. Receiving one of her letters, Cushman was, for once, tongue-tied: " What though I am kindly bidden-" she stuttered, "what can I offer in exchange?"

What Cushman could offer to the many women (and a few men) who entered her charmed circle was, in Markus's view, a model of dedication to work and to intimate friends that inspired a generation of female artists. She filled her mansion in Rome with conspicuous talent: among others, the plucky and puckish sculptress Harriet Hosmer and the even more talented Emma Stebbins, who designed the lovely Bethesda Fountain in Central Park. Henry James relegated this "white marmoreal flock" to the background of his biography of a safely married expatriate American sculptor, "William Westmore Story and His Friends." A major aim of "Across an Untried Sea" (a title drawn from a newspaper verse about Columbus) is to nudge aside the bust of Story and replace it with Cushman's "blunt, no-nonsense face." Too long dismissed as spinsters or eccentrics, "neither Charlotte Cushman nor her circle of friends," Markus amply informs us, "forgot to live". Bette Davis should have played her in the move.

Christopher Benfey teaches English at Mount Holyoke College. He is the author, most recently, of Degas in New Orleans.