Fissures Under the Crust : BLACK DOGS, By Ian McEwan (Doubleday: $19.50; 149 pp.)

"It wasn't his red flag, you know," Bernard says the next day. "I don't think they even saw it. You heard what they were shouting? Foreigners out!"

In the novel's preface, McEwan's narrator outlines the reasons for his obsession with his in-laws, calling the elder Tramaines "the extremities, the twin poles along whose slippery axis my own unbelief slithers and never comes to rest." But his obsession has a more personal cause as well. Orphaned in childhood, he is acutely aware that all children are in a sense a reconciliation of opposites, a synthesis between two points of view that in his case remain unknown. "Nor will it do," he warns, "to suggest that both . . . are correct. To believe everything, to make no choices, amounts to much the same thing, to my mind, as believing nothing at all."

By the book's end one thing is certain to rationalists and mystics alike: June's dogs come between people. Forty years later they're still causing arguments among neighbors in a French village. Was an earlier victim of the dogs--when their German masters were still on the scene--an informer, as some of the villagers suspected, or a pretty woman whose aloofness offended her male neighbors? The arguments, like the schism in a marriage, go way beyond a single incident or a pair of beasts, go beyond intellectual opposites to fundamental polarities including gender. And McEwan's preoccupation with menace is revealed as part of a larger preoccupation with humanity.