Contemporary poets test new dimensions; A Coast of Trees, by A. R. Ammons. New York: W. W. Norton & Co. 52 pp. $4.95 (paperback).

By Victor Howes; Victor Howes, who teaches English at Northeastern University, is author of ''The Lobsterman's Daughter and Other Poems.'' Poet, professor, and prizewinner (he has a wards from both Bollingen and the American Academy of Arts and Letters), A. R.Ammons is now writing some of his best poems and some of the best poems being written today. Reading his poems is like taking a nature walk -- a cosmic nature walk. Ammons looks like a nature poet; he's always talking about hedgerows and grackles and brooks and snowstorms, but don't let him fool you. He's after bigger game. Ammons feels his tenure upon earth -- all our tenure -- is illusion, ''as insubstantial and permanent as a mirage.'' Oh, we have our macadam and our concrete, our aunts and our uncles, but watch out, ''reality and appearance flow away together,'' and this is just a place, this earth. deep Space begins at our heels, nearly rousing us loose: we look up or out so high, sight's silk almost draws us away: this is just a place. . . We're here and soon gone, in fact sometimes a whole green sunset will wash dark as if it could go right by without me. John Ashbery is another kettle of fish. Flying fish, doing somersaults in midair, and all sorts of sleights-of-fin-and-fun. Yes, of course Ashbery has starlight in his poems, but it's ''a swift nightmare of starlight on frozen puddles in some/Dread waste.'' And there's a mouse, a city mouse with a passport and its color photo in it. There is also Job, sitting ''in a corner of the dump eating asparagus,'' and there are ''angels elbowing each other on the head of a pin. Ashbery is always familiarizing the more-or-less unfamiliar. ''Trust me,'' he confides. ''The world is run on a shoestring.'' But what a shoestring! Ashbery's world, like Ashbery's poems, is absurdist, surreal, endlessly surprising. Sentences typically run away with themselves. Example: ''The endless ladder being carried? Past our affairs, like strings in a hop-field, decants/A piano tuning we feed on as it dances us to the edge.'' No wonder critics have called Ashbery opaque. His subject is the disappearance of subject matter. His favorite literary device is the non sequitur. His book is titled ''Shadow Train,'' and what is a shadow train? A train of shadows? A train in the shadows? Or one train programmed to shadow another train, like ''the endless ladder being carried past our affairs, like strings in a hop-field''? In short a train and not a train at all? It's anybody's guess.

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