The northern half of Brazil, taken up by the 2 million square miles of Amazonia, remains a world of its own. At the headwaters of the rivers that feed the Amazon, tribes whose hunting, gathering and rudimentary agricultural existence has changed little in the last 10,000 years are still being discovered. In the territory of Rondonia, 5,000 poor whites and several hundred Indians have been clashing since last September over land supposedly granted to the Surui tribe. "It's like a bangy-bangy [a Hollywood Western]," a knowledgeable Brazilian told me. "You've got the good guys, the bad guys and the Indians."

Yet in Manaus, men in suits with attache cases step off planes, do their business and fly home the same day, as in any modern city, and there are strips and subdivisions which look no different from southern California. The new echnological wave has made some real impact in Amazonia - The fabric has not yet been destroyed but, as with a moth which has just begun to attack a fine piece of silk, a few big holes have been made.

The biggest holes are in the state of Para, where hundreds of thousands of acres of forest have been cut down and burned off to make pasture for cattle. The desirability of these fazendas - ranches - is questionable: On the one hand, Amazonians will be wanting more meat as their standard of living improves and they can afford alternatives to the poor man's staple of rice and beans. On the other hand, most of the soil will turn to brick-solid hard-pan where nothing will grow - "green hell to red desert," as one group of scientists describes the transformation.

What is happening in Para is a hit-and-run operation. Many of the Fazendeiros are clearing more land than they will get around to using. There are laws enjoining them to leave 50 per cent of their forest holdings standing and to use the wood they cut, but as with most of the rules controlling what is done in Amazonia, there is no enforcement, and nobody pays much attention to them.

Some Amazonians look at the operation of the reclusive American billionaire and oil tanker tycoon, Daniel K. Ludwig, on the Jari River as the bright light of the future. Ludwig's property is bigger than Connecticut. His rice is doing splendidly, and Amazonia could become "another Mekong Delta," one nutritionist enthused. Rich deposits of kaolin are already being mined and the rare clay shipped to China-glazers around the world.

But most of the land is in plantations of Caribbean pine and a fast-growing Asian tree called gmelina, and the ecological future of a monoculture - the same species of tree planted in rows - is uncertain in Amazonia. Henry Ford learned this in the 30s when he tried to revive the rubber boom and built a huge plantation called Fordlandia on the Tapajos River. By Planting his rubber trees all in a row, he was only setting them up for their natural enemies.

A few seasoned Amazon-watchers have their doubts about the Jari project. Ludwig is in his 80s, and one observer speculated that his heirs will lose interest in the operation. Jari has yet to prove itself, although reportedly Ludwig has already sunk $180 million into it.

Five years ago the byword in Brasilia was "national integration," and a system of highways running up and down and around the Amazon basin, linking places previously accessible only by river or air, was begun. Today the road from Manaus to Boa Vista is open; you can drive from Cuiaba to Santarem; and the Perimetral Norte, along the northern rim of the basin, has gotten as far as the country of the Yanomami, Amazonia's largest and most primitive indigenous group. The Transamazonia has been halted at Humaita where a road crew hit emeralds.

But the hopes for settlement along the Transamazonia have not materialized. Only one-tenth of the people expected to migrate from the poverty-ridden Northeast have shown up, and many are involved in messy disputes over the ownership of land hitherto been considered worthless. Last August, an American farmer and his sons were gunned down by squatters on his particularly fertile spread near Maraba.

When the roads are completed, the Amazon will be a smaller place. But for the people living like pioneers in the back-country, a week or two by canoe from the nearest store, life will remain the same. Stinking and oozing with life, Amazonia is a brutal environment for humans, and this is why it remains the last vast wooded wilderness on earth.

But there is little question that this civilization can in time subdue it. Unlike earlier efforts which melted into the jungle without a trace, this one seems to be here to say.No place on earth, however wild and forbidding, is strong enough to withstand it.

THE PEIXE-BOI, or manatee, is one of the many amazing creatures living in the Amazon. An aquatic relative of the elephant, it grows to be several tons and propels itself along with graceful flicks of its tail fin. Its blimp-like shape and the other details of its anatomy make it perfectly adapted to munching placidly on grasses, hyacinths and other plants in the lakes, shallows and quiet backwaters of the Amazon system.

Pexie-bois are traditionally hunted by mariscuadores, the subsistence Amazonian fishermen who live along the tributaries of the tributaries. The mariscuadores harpoon the animals at night. The point of the harpoon detaches from the shaft and is connected by a chord to a wooded buoy which the mariscuador clutches to his belly as the terrified creature pulls him and his canoe along in the attempt to struggle free. Before long the exhausted peixe-boi rises to the surface, and the mariscuador administers the coup de grace by stuffing its nostrils with wooden plugs. The peixe-boi is towed home, quartered and fried in its own fat. Its flesh tastes like ham and, stored in 20-liter kerosene cans, keeps for years.

But the mariscuadores complain that, while they used to bring in five peixe-bois in a night, they are lucky now to find one in a week. No one knows how many of the animals are left; one scientist ventured "a ballpark estimate of 10,000" for all of Amazonia.

No wonder the peixe-boi is dying out; like the jaguar, the giant river turtle and the anaconda, it is usually killed by rural Amazonians on sight. A peixe-boi will provide meat for weeks; a turtle or its eggs offer a welcome change from fish and farinha; a jaguar or an anaconda skin provides something to trade to the regatao, or waterborne merchant, for cloth, coffee, cartridges and other necessities.

The mariscuador himself is a vanishing breed. While it takes a certain skill to throw a harpoon, anybody can now stretch out a gill-net and catch five times as many pirarucu, tambaqui and catfish as a mariscuador can. The trouble is that 50 to 60 per cent of the gill-net catch is usually lost. The fisherman usually works 20 nets. A fish caught in the net dies in 15 minutes and, because of the highwater temperature, begins to grow soft and to stink in half an hour. Those fish which aren't spoiled are often devoured by piranha.

All over Amazonia the animals are being wasted at a terrible rate. Millions of brilliant red cardinal fish are netted yearly at the mouths of the creeks which empty into the Rio Negro; they are destined for American aquarium-lovers, but only 10 per cent of them reach Manaus alive. In the high jungles of Peru, where there are more species of butterflies than anywhere else in the world, many of the local people live by guiding foreign collectors and selling the insects to people who make Quechua Indian is paid a few dollars for a poached vicuna hide worth several thousand in Europe. Everywhere monkeys are sought for food and sale to zoos, medical laboratories and pet shops.

THE MOST effective conservationists in the Amazon are the Indians. They are the region's largest landholders, although land granted them "in perpetuity" has repeatedly been sold out from under them. The forests on Indian land are invariably intact and their animal populations thriving; an Indian will never kill a young creature or an expectant animal mother, or overhunt a particular species, or take more than he needs. Other people stay off their land because they know what the Indians do to interlopers. One has only to look at southern Brazil, where the only primary forests left belong to the Indians, to see how well land fares under their management.

Anyone who has spent time with an Amazonian tribe cannot avoid being struck by the beauty, skillfulness and freedom of their existence. As a mature adaptation to the rain forest, their culture offers unique insights into the whole ecosystem. Much of what the rural white Amazonian does to survive - his cultivation and processing of manioc, his harvesting of Brazil and palm nuts, his use of the dugout canoe and timbo, the fish-stupefying vine - was learned from the Indians. They have already given us curare, and some enterprising graduate student might analyse the many plants they use for birth control and discover among them the safe contraceptive which our society still lacks.

One wonders who would want to change people who are basically content - riddled with disease but content - and force them to become part of a restless, modern society. But this has been the stated aim of the Brazilian government. "We are going to create a policy of integrating the Indian population into Brazilian national society as rapidly as possible," Rangell Reiss, minister of the interior, said in 1974. "We think that the ideals of preserving the Indian population within its own habitat are very beautiful ideas, but unrealistic."

What happens to Indians who are thrust directly from the Stone Age into the 20th Century is one of the great tragedies of Amazonia. "Amazonia," Cludio Villas-Boas, one of Brasil's leading Indian experts, has said, "is full of them - witless, detribalized remnants of a once vital people, passive morons of civilization waiting for decisions to be made in their behalf."

I didn't meet any of these "passive marons," but in Iquitos I heard about jungle lodges to which tourists are taken to meet some real, live Jivaro Indians. The tour director tacks a ten-sole note to a tree and the first Indian who hits it with a dart from his blowgun gets to keep it. Deeper in the jungle, unacculturated Jivaros are still raiding each other's villages, carrying off the women and shrinking the heads of their male victims.

FUNAI, the National Indian Foundation, recently put a stop to an outfit called Green Hell Tours which had been taking boatloads of tourists from Laeticia, Colombia, to a Tucano village in the Brazilian Amazon to see a ceremony called "plucking of the virgin's hair."

Even more serious than the loss of identity from too abrupt or exploitative acculturation are the diseases that the white man brings. The Indian has no resistance to flu, pneumonia or tuberculosis and, once he is smitten, he often decides that the evil is too strong to resist and lies in his hammock waiting for death. When a whole village is seized with an epidemic, the food-gathering mechanism breaks down, and more people die of starvation than of the disease itself. According to one estimate, Brazil's indigenous population dropped from 250,000 to 80,000 between 1963 to 1968, years when many new tribes were first contacted.

But each culture, like a star, has its own life expectancy. You can't keep it alive or isolate it artifically any more than you can pen up a restless teenager who wants to see something of the world. Many fo the Indians I met who had been slightly exposed to civilization wanted to know more: they wanted to see Manaus and ride in a plane and meet the women they'd been hearing about. I sensed they could take care of themselves; many Amazonian Indians have, in fact, become experts at manipulating civilizados (myself included) into getting what they want. There is a side to them which they never show to the white man. Far from being forgotten, their culture and the lessons they learned from the jungle act as powerful spiritual forces that enable them to keep things in perspective the deeper they get into our way of life.

"There is no such thing as an integrated Indian," the FUNAI director, Gen. Ismarth Afaujo, remarked recently. "There are Indians who are in permanent contact with society, but that does not mean that they are integrated."

WHAT is it like in the jungle? In the first place, it is not a dead or passive environment like a city.It is impossible to avoid becoming a part of it. The minute you step into the jungle you are caught up in the food web: you become the prey of hordes of stinging insects, and within a day you have had to reconstitute a bird or a deer or hack down a tree for the berries you need for sustenance. You take your own medicine with you and administer your own injections. You body acquires a pervasive smell, the sweet stale stench of an organism constantly exuding fluids. Pockets of fat melt away. You become lean and hungry - always hungry, and there is never enough food to satisfy your hunger.

Your senses become attuned to every sound, smell and movement. No other environment is so intense or exacts such total sensual involvement. All around you are vines that look like snakes (plus a few snakes taking advantage of the situation by imitating the vines), moths posing as hummingbirds, orchids mimicking bees, butterflies looking like leaves. It is a big game, with an infinite number of players employing an infinite number of strategies to be able to keep playing it.

The longest time I spent in the jungle was seven days in the terriroty of Roraima, traveling maybe 150 miles from Jose Peruano's hut on the Rio Catrimani to the highway to Boa Vista in the company of three Yanomamo Indians. During that time, we lived on fruits and nuts and the game which Leonca brought down with his 7-foot arrows. We saw the sun only twice, once from the rocky crest of a peak and once when we emerged from under the trees to cross the Pacu River.

Ruby, Manuel and Leonca moved at a pace between a walk and a run, slipping between vines and branches along a dim path that was only 6 inches wide and often invisible to me, putting each foot directly in front of the other and grabbing the ground with their spread toes. At night, the three of them would swing in their back hammocks and engage in a rhythmic dialoge called wayamu for hours. At dawn, they would roll tobacco leaves in the ashes of the fire, stick them under their lower lips and take off.

Leonca was one of the most respected shamans in the region. I met him on New Year's Eve when he walked into the Malocca, or round house, inhabited by the 30 remaining Yanomami of the Aika who had not been carried off by tuberculosis. He was about 40 years old, with the wide-open expression of a child and the perfectly toned body of a dancer. He had a bowl-shaped haircut and wore nothing except a string around his waist and thin strands of white beads around his neck and upper arms.

Leonca laid down the bow and arrows which dwarfed him by a foot and a half, opened a quiver containing the needle-like points used to kill monkeys and began to rub the points against each other over a banana leaf until a small pile of reddish-brown powder had collected beneath them. He raised the powder to his nostrils and snorted it, four times into each nostril. Then he retired to his hammock. Fifteen minutes later he got up and started to chant, then spat, and moaned, and danced with great sweeping movements of his arms and imploring looks to the sky. The chanting was in a beautiful minor key like a spiritual. Long strings of strange syllables cascaded from his mouth. "Only he knows what he's saying," Ruby told me.

In the morning, I offered Leonca a T-shirt, a pair of underwear and a flashlight if he would come with us. He agreed, and the next week was filled with images of Leonca swarming a 60-foot warok tree and throwing down delicious blue berries, moving swiftly and soundlessly with drawn bow through the underbrush toward a plump black curassow.

The first night, Ruby came back with a jacaretinga, a small alligator, hacked off its tail with his machete and put it on the fire.The tail twitched and thrashed in the flames for a long time.

The second night, we ate jaboti tortoise. Ruby ripped off the animal's plastron. The heart was still jumping. The tortoise was still alive, but the idea that it may have been in pain was not something that would occur to Ruby, conditioned since childhood to ignore pain in himself.

THAT NIGHT I felt as if I had accomplished my goal in coming to the Amazon. I had wanted to "get lost in the jungle, to leave my civilization and taste the world pure and uncorrupted by its influence." Well, I was certainly lost in the jungle. No one knew where I was and, if anything had happened, they wouldn't have known where to begin looking for me. I was in another, earlier time where people were still an integral part of nature and not living in an artificial world apart from it. I had found in Leonca a natural man in harmony with himself and his environment, and I could feel through his gentle radiance his sense of oneness with all the living things around him.

"Look at these ants," he said the next morning as we sat resting. They are saubas, or leaf-cutters. Thousands of them were hurrying back and forth along a long, clean 4-inch wide highway which they had cleared in the leaf litter. Some were coming down from a big tree carrying shredded leaf flakes many times their size to a mound of red earth several yards high and maybe 30 yards in circumference. Others were returning from the mound and going back up the tree. The leaves were being taken to subterranean gardens inside the mound where the ants used them as compost to grow the fungi on which they subsisted.

"The have their own highways," Leonca laughed. "Maybe they will take you to Boa Vista."

Try as I might, I never could stop the internal monologue, the broken thoughts of a thousand shapes, none of which had anything to do with where I was, that ran through my mind as I struggled to keep up with Manoel, Ruby and Leonca. In fact, as the trip wore on, the trappings of civilization became more important than they had ever been for me. Perhaps I fell back on them as a defense against the overwhelming wildness with which I was surrounded. I became obsessed with keeping my fingernails clean and my face shaven, and I lived for the period in the afternoon when the others went hunting and I could curl up in my hammock with my one precious book. It came as a bitter disappointment when I realized that even here, deep in the Amazon, I was a prisoner of my own culture.