PETER MATTHIESSENS last book, The Snow Leopard was less an account of his trip into the Himalayas looking for a legendary mountain animal than an exploration of himself, seeking some meaning in his troubled life. Now in Sand Rivers, he has cleared himself of introspection and written as extroverted account of a trek into one of the last primeval placed in Africa, the Selous. He gives us all the expected bits of safari lore -- the unwary animals, the close calls, the naive native carriers, the sardonic old Africa hand who greets the American writer sceptically and ends up respectful and friendly. Of course, since it is Matthiessen who is telling all this, Sand Rivers has -- as his books always do -- a special grace of perception and an understanding of nature made up in equal parts of love and learning.

The Selous Game Reserve in Tanzania is 22,000 square miles of wilderness, four times the size of the famous Serengeti preserve, as big as Maryland. Poachers get in around the edges, taking elephant ivory or rhino tusks (to be made into aphrodisiacs for the Chinas or, dagger handles for OPEC princes). The Selous water holes are jammed with hippopotamus, the grasslands dotted with antelope, the trees and skies full of tinker birds, boubous, griffons and hammerkops. The air sounds with the "tinny notes of the trumpeter hornbill," "the deep tearing coughs of a restless leopard," the chirping of millions of frogs "that hurl their voices at the stars in a bug-eyed cosmic ecstacy." The streams are bedded with a white quartzite, hence sand rivers.

Matthiessen and photographer Hugo van Lawick went to Selous as publicity agents (of a kind) to call attention to a last unspoiled piece of Africa and save it from neglect or the fate of the game parks, tamed by Land Rovers and Minibusses, where animals accept men because they are accustomed to them. In the Selous animals accept men because they have not known them.

Their book is an evenly told story, without sermonizing or any real climax -- and it eloquently accomplishes their goal.

Almost mischievously, Matthiessen quotes a journal kept by one of the porters who summed up the trek: "We went up and down hills and met some different animals." Some animals!

As a waterhole where hippos were staying under water "cooling the cumbersome machinery of their brains, . . . two pink eyed gladiators" emerged "with a quake and rumbling . . . reared up on their hind legs, mouths wide and ivory clacking; their huge heads locked." The party stumbles head on into a rhinocerous with her calf: "The immense and ancient animal remains motionless . . . I am in pure breathless awe of their protean life form, six hundred thousand centuries on earth, the ugliest and most beautiful life imaginable." But the rhino simply twitches an ear and "settles backward inelegantly on her hind quarters." The moment of purest excitement seems to come when the expedition leader takes Matthiessen into a hidden clearing and tells him "you're the only white man besides myself ever to see it."

The leader, a tall and sometimes truculent man named Brian Nicholson had been warden of the Selous until the hard-pressed government could no longer afford proper care. In his day he had shot rampaging elephants at point-blank range and killed 20 man-eating lions. He seems at first a colonial throwback, as scornful of imported scientists -- "boffins with their great pulsating brains selecting facts to fit their precious theories" -- as of the native Africans. But he himself turns out to have done pioneering research on elephants and for all his prejudices feels, like other old hands, that civilization is turning first-rate Africans into third-rate Europeans. The Selous is "the only place on earth where I feel I belong" and he aches to save it.

Hugo van Lawick's photographs are lovely and revealing and, though they show familiar animals, are never perfunctory. They serve the book best when they evoke the innocence of the Selous -- elephants ambling through a blue-white shallows, antelope skittering into a golden haze, water bucks placid in a violet dusk. Matthiessen's text itself is touched with a photographic immediacy: harrier hawks "with bare vulturine lemon-yellow faces" that go off in "a strange weary flight through the bony trees"; baboons that "descended stiffly from a tamarind and moved off like old men": zebras "fleeing like striped spirits through the trees."

A famous photographer once remarked that, for him, the ideal camera would be a lens screwed into his forehead and focused by his brain so that he could take pictures without any intervention. In a sense, Matthiessen, the writer, is equipped with that ideal camera. The things he sees are captured with the click of a thought on his mind and later fixed and printed by a prose rich with specific and poetic imagery.