An unabashedly sentimental safari; Sand Rivers, by Peter Matthiessen, with photographs by Hugo van Lawick. New York: The Viking Press. $19.95

The author of the award-winning book "The Snow Leopard" now takes us on a safari to perhaps the largest and least-known game preserve in Africa. This jungle hideaway -- the Selous reserve in southern Tanzania -- is fascinating in its immensity, its remoteness, and its animal denizens, some of which never before have encountered man.

But what makes the safari come alive is Peter Matthiessen's keen eye for the flora and fauna of Selous. He has an unusual ability to describe them in their natural habitat. He coveys vivid word pictures that will bring a nod of recognition or appreciation, or perhaps a jolt of surprise, to those of us whose game viewing in Africa has been confined to three-hour conducted tours from a tourist lodge. He is especially brilliant with such beasts as hippo, rhino, and elephant -- and he never fails to note the birds while others are watching only for animals.

The white men on the safari include Baron Hugo van Lawick, whose photographs complement the Matthiessen text, and Brian Nicholson, the vastly experienced professional hunter and safari leader who knows the Selous well. Although taciturn at times, Nicholson is just the type most of us would expect to lead our expedition into the wilderness.

Selous was named after Frederick Selous, once a white hunter for Theodore Roosevelt at the turn of the century. The current safari set off in 1979 to find large wild animals in quantity in one of their last strongholds. The aim, however, was to see animals, not shootm them. With big game being slaughtered so rapidly, the day of the vast herds already is nearly gone, and this may be the last generation to see parts of Africa as they have been for centuries past --namely, swarming with game.

For those interested in wildlife, "Sand Rivers" is a day-by-day chronicle of what a major jungle is like, and what animals and humans must do to survive there. Thanks to the encroachments of land-hungry Africans determined to turn the jungles into fields for food and of poachers bent on killing the denizens for their ivory or pelt, neither terrain nor beast can do well today except in such isolated areas as Selous.

Some of the book's best moments are when Nicholson reminisces in camp at night about his early days in Selous or explains on the trail what the animals nearby are doing. Rivers and pools are, as ever, the focal point for thirsty animals, but the sand rivers are those without water or down to a few precious inches in pools here and there, glinting in the sun but lacking much of their essential ingredient for wildlife. Yet their smooth sand can provide pathways for the feet and vehicles of man's safaris to penetrate to the very heart of a hinterland.

Matthiessen, Nicholson, and van Lawick not only see animals at close range, but they see them in minute detail. Elephant, one learns, sometimes graze with their eyes closed completely "or else look down at the ground at their feet, when all one can see of the eyes on close examination are the top eyelids and the beautifully long eyelashes." Hippo are great bluffers but can be dangerous and fast despite their size. Water buffalo are very unpredictable and fearless, and other big animals usually give them a wide berth.

Matthiessen notes that the safari often trod well-worn trails two feet wide and adds that such paths "have never been seen or walked by human beings." That makes you think of Africa's antiquity and vastness. Nicholson in a glum moment says of the Selous, "I suppose it's a mistake to revisit a place you loved, to make this sort of sentimental journey . . . . A place can change a lot in 16 years.

Matthiessen speaks of the jungle's deep silences -- and its occasional vast noise. "A hyena whoops and another answers . . . . Then a shocked ringing silence as if the night hunters have all turned to hear this noise." A heron catches a frog and suddenly other frogs go mute. "But soon an unwary one . . . tries out its overwhelming need to sing out in ratcheting chirp; another answers , then millions hurl their voices at the stars." That's wilderness. That's Africa.

Nicholson (the porters call him Mr. Meat because he shoots their food) is described as a "bad old bush rat" with unshaven jaw, bad smile, and loose teeth. He says: "In Africa, out in the bush, man is still a part of nature and what he does is mostly for the better. It is only where the bloody Western civilization has come in that everything is spoiled."

They stumble onto a rhinocerus with calf at her side. Will she charge? Matthiessen calls her "the ugliest and most beautiful life imaginable" and the calf, "staring with fierce intensity in the wrong direction, is of a truly marvelous young foolishness." Not everyone sees all this in a rhino that decides not to charge and finally lies down in a nearby bush. But, "That was worth the whole safari," Matthiessen says.

So things go on this unabashedly sentimental foot safari into Selous. At the end, no one says, "Dr. Livingstone, I Presume." But Brian Nicholson sums it up when he declares the old safari days are over. "In Kenya, people just jump in their Land Rovers and minibuses and combis and away they go, but there really isn't any place left to go to.

"I saw the last of it up there 20 or 30 years ago," he adds, "and the Selous is the last of it down here, make no mistake."