SOUTHERN LIGHT. By J.R. Salamanca. Knopf. 675 pp. $19.95.

J.R. SALAMANCA IS a gifted, thoughtful and accomplished novelist whose three major books -- Lilith, A Sea Change and Embarkation -- have been deservedly praised. His fiction is usually set on and about the Chesapeake Bay, which he describes with deep love and knowledge; indeed, more than any other contemporary writer of fiction or nonfiction, he has made the bay his own. His characters have intense interior lives, into which he draws the reader with great feeling and sympathy even when, as is sometimes the case, the characters are self-destructive. At their most effective and beguiling, his novels occupy a territory somewhere between reality and fantasy, a place where both actions and emotions are often larger, more charged with import, than we find them in ordinary life.

All of these characteristics of Salamanca's work are present in Southern Light, his largest and most ambitious novel, but here he employs them to disappointing effect. As one who admires and values his work, I take no pleasure in reporting that Southern Light is a failure: a grand failure, perhaps, but a failure all the same. It is too long by, quite literally, several hundred pages, too crammed with irrelevant and unrevealing detail, too portentous and pontifical. There can be little doubt that Salamanca intends the novel to be his magnum opus, but like other novels written with such high hopes -- Faulkner's A Fable and Hemingway's For Whom the Bell Tolls lurch into mind -- it is loaded down with more weight, symbolic and otherwise, than it can bear. A question asked by one of his characters could be directed at Salamanca himself: "You're getting quite a lot out of your system, aren't you?"

As the novel's epigraph Salamanca takes some lines from Keats' "Ode on a Grecian Urn," and as his principal theme he takes that great poem's contemplation of the relationship between truth and beauty. The seeker of truth is Carl Ransome, a widower and retired physican now living on Solomons Island at the mouth of the Patuxent River, while that of beauty is Sylvie Linthicum, a lovely woman half his age who is, as Salamanca followers will quickly recognize, the daughter of the boatwright who is the protagonist of Embarkation. Each has come to the island in a mood that mingles grief, guilt and spiritual bewilderment; each soon turns to the other for companionship, platonic love and comfort.

The beginning is promising: a long, lovely passage describing the island and Carl's reasons for coming to "the great voracious cleanliness that reigns here at this end of the world," followed by an account of the slow awakening of friendship between these two people who seek nothing so much as solitude. But within 50 pages te novel veers off into extended flashbacks, separated by nothing more than about an inch of white space, in which Carl and Sylvie talk to each other about their pasts. More accurately, we are intended to believe that they are talking to each other, but it is quite impossible to do so because the tone of the flashbacks is literary rather than conversational, because most of the flashbacks are far too long to be convincing as conversation, and because the amount of extraneous detail they contain is quite simply stupefying. Here is a too-characteristic passage, from one of Carl's narratives:

"I closed the notebook and went into the bedroom and undressed. There was no closet in the room, but an ancient Victorian wardrobe, whose creaky, swollen doors would not close tightly. I hung my clothes in it and went into the bathroom to brush my teeth. While I did so, I watched a horde of small black ants dissecting the body of a cricket that lay belly-up on the linoleum behind the toilet stool. I replaced my toothbrush in the water glass, plucked up the cricket and its scavengers in a wad of toilet paper, and flushed them down the toilet. I washed my hands, went back into the bedroom, switched off the light, and lay for half an hour on the damp and lumpy mattress without being able to sleep. I switched on the bedside lamp, went down the hall to my improvised office, and brought back a textbook from the shelf beside my desk. I lay down again on the bed and, opening the book at random, began to read."

THE DETAIL in that passage is numbing, but even more so is that none of it means anything; it is simply there, droning along to no evident purpose, until a great mass of nearly 700 pages has been accumulated. When Carl tells about his years in Alabama during and after World War II, when he worked on a syphilis study that bears obvious resemblances to the infamous Tuskegee project, he describes his researches at such length that one feels immersed -- for n apparent reason -- in a medical textbook; when Sylvie talks about her lovers and her uncompleted novel and her strange, haunted family -- not to mention her seemingly endless trip to Mexico -- she goes on and on to a degree far greater than is necessary for the reader's knowledge or pleasure. As for the conversations between the two about truth and beauty, good and evil, omission and commission, guilt and redemption, blindness and light, and other such matters, they have approximately the urgency, and too often approximately the weight, of late-night collegiate bull sessions.

Southern Light is certainly a serious book, one that means to say important things about important matters; its seriousness, like its prose, commands respect. But it cries out for extensive pruning and reshaping; inside this fat book is a thin one struggling to break free. The skills of a good editor are never to be undervalued, and Southern Light is desperately in need of them: not merely to cut it down to a length manageable for author and reader alike, but also to solve its critical problems of voice and point of view and to give it a more coherent narrative line than it now has. As often happens with books in which authors have invested vast amounts of time, emotion and labor, Salamanca may have gotten too close to Southern Light to appreciate that its virtues -- intelligence, inquisitiveness, compassion, plain decency -- are smothered by its bulk, its accumulation of pointless detail, its sheer windiness. The story of Carl Ransome and Sylvie Linthicum, two people who find redemption in confession and love, has the potential to be a powerful one, which makes it all the more a pity that Southern Light falls so far short of what its author can do, and has done.