A flurry of ocher-colored dust kicked up at the top of the hill, and behind it a horse-drawn cart came, bumping its load of settlers down toward the center of town. Smoke still curled from the smoldering jungle beside them.

On the wide dirt streets there was a stirring now, just after daybreak, when the heat had not yet become a weight against the lungs. The barber laid out straight razors at the Ideal Salon. A boy at the general store raised shutters to let early light in over the spurs, the hammocks, the ax handles and lanterns and canned green beans. Lean-faced men in cowboy hats pedaled their bicycles down toward the main street, their black balloon tires leaving single tracks in the settling dust.

While the United States was celebrating its 1976 bicentennial, this wood and red-earth city was pure jungle -- a thick wet tangle where three-toed sloths hung off the high branches and vine-draped trees grew 30 times as tall as a man. Within five years, 75,000 Brazilian pioneers have settled in Ariquemes and the hacked-out crop land nearby it to reap the celebrated promise of the southern Amazonian territory called Rondonia.

This is the Third World's Wild West, a territory snatching homesteads and minerals out of the jungle for stakes that history never forced on North America. With its creaking river ports, its dirt city streets that turn to red mud after the rains, and a swath of good soils to grow foodstuffs and rubber, Rondonia is Amazonia's boom territory -- the southern flank of an unprecedented Brazilian assault on the greatest river system and tropical rain forest in the world.

The settlers have burned jungle and then slashed at dead tree limbs with machetes to clear land enough for planting rice and manioc and black pepper. They have chopped and gathered logs of jungle hardwood to nail together the rough wood buildings for their soda fountains and movie theater. They have left behind farm workers' shacks in the southeast, drought-ravaged fields in the Northeast, subsistence jobs in the great Brazilian cities that have no lure of jungle frontier.

There is a young dark-haired Rio man who calls himself Carioca -- the equivalent of moving to Wyoming and calling yourself "Baltimore" -- and serves his wife's grilled meat and rice and beans on the sheltered outdoor platform he built himself, with a perch for the parrot. There is a young Japanese data processer from Sao Paulo, waiting now for his promised plot of farm land, who sings love songs at night in Japanese when he has drunk enough sugar cane liquor, and then opens his hands to strangers to display his new blisters from planting his garden. "These are what I want," he cries.

There are gold miners sleeping in hammocks slung across their floating river dredges, booster politicians lobbying for Rondonia's statehood, single-minded pioneer families who came too late for good government land and settled, armed with shotguns, on the land of Indians whose first peaceful contact with white men came less than 15 years ago.

And still the people keep coming, sometimes a thousand in a single month, climbing out of dusty overland buses in the block-long open-air terminal whose official name, inscribed on the wooden walls, is Immigrants Station. A small blue ticket comes with every meal at the station's lunch counter, and on each ticket, someone has printed in black letters across the top, "Always serving the pioneer better."

Nobody knows with any precision the extent of the resources that lie within the boundaries of the Amazon basin. But even the rough estimates are staggering: in Brazil alone, a trillion dollars worth of hardwood; 10 billion tons of rock salt; a single 18 billion-ton iron ore deposit that is apparently the largest in the world; enough gold to make Brazil the world's third largest gold producer; one-seventh of all known bird species; more fish than can be found in the Atlantic Ocean; more than half the world's tropical forest. And under all that forest, there is 2 million square miles of land -- a river-fed basin that is one-twentieth the surface of the planet earth.

"Amazonia is the last place in the world where man hasn't taken over yet," said Breno dos Santos, the Belem geologist who in 1967 was credited with discovering the massive Amazonian iron-ore deposit that may help reshape the future of Brazil. "After Amazonia there's only Antarctica. And in world history, when man goes into a region like this, he always goes in destroying the land and killing other men. He's never gone in using his head. This is the last chance for man -- he gets a chance here to show either that he's no longer a child, or that there's no more chance for man."

To all but the indigenous people who understood survival within the Amazon, it has for three centuries been a hot, humid, inhospitable place that beat men back as much by its natural fragility as by its ferocity. It swallowed explorers, or gave them lifelong fever. Grand agricultural efforts withered within its poorly understood ecology. Its flourishing rubber trade built a flamboyant riverside capital that sagged with the coming of cultivated Malaysian rubber.

Stretched around the 4,000-mile-long Amazon River, the basin that makes up Amazonia is an intricate network of river tributaries, dense jungle, flood plain and warm savannah. Its terrain stretches into eight South American countries, and some, like Peru, are already harboring ambitious development plans of their own.

But the overwhelming bulk of Amazonia lies in Brazil, the fifth largest country in the world, and Brazil has never needed a frontier as it does now.

Its remarkable industrializing economy, which grew by 10 percent a year and more in the last decade, has suddenly ground down to 3 percent. Its official inflation rate is one of the highest in the world -- 110 percent annually. Its millions of landless peasants are swelling urban slums and invading the land of larger private owners, sometimes with violent results. And almost every time its Cabinet ministers go abroad for their now-legendary financial machinations, the country gets further in debt.

The Brazilian debt has by now become something of an international symbol for the pitfalls of modern development. It is the largest foreign debt in the world -- $60 billion is the most common estimated total of public and private long-term debt, with another $6 or $7 billion in short-term debt. Major lenders all over the world, public and private, have such massive amounts of money tied up in Brazil that it is generally assumed a Brazilian default would imperil the world's banking system.

The pithiest summary of the financial world's passionate interest in Brazil comes from a Brazilian banker who is fond of saying, "If I owe you $10,000, I don't sleep at night. If I owe you $10 billion, you don't sleep at night."

So Amazonia, with its resource wealth and 60 percent of the nation's land, has become the new Brazilian frontier. Fifteen years ago, following a coup by generals who vowed to build up "national security" and work a revolution in the country's economic development, Brazil began tearing into the jungle with Amazon development plans bigger and more audacious than any the country had ever tried before.

"It's our moon project," a Brazilian official told an American reporter at the six-year mark. "It's like the moon, but far more valuable to us. That's why we had to do something about it, whatever the cost."

They were short on research, planning and geography. On their maps, rivers sometimes were registered 20 miles away from their actual course. They knew so little about the detailed contours of the jungle that road crews were astonished to find thousand-foot-high hills on what they thought was flat wetland. Nobody set out to examine precisely what the jungle soils would do when men tried en masse to coax corn fields or cattle pasture out of them.

But Brazil is famous throughout South America for the scope and fervor of its great projects -- this is the nation that built a swooping, futuristic high-rise capital in the middle of an empty red-dirt plain. And that is how they entered Amazonia, with plans so grand that the international press began to pay attention.

Road crews plunged straight into the jungle to carve a 3,500-mile highway straight across Amazonia toward the Peruvian border. Ranchers and industrialists were invited to bring their bulldozers in to clear the way for beef cattle. Colonization officials spoke enthusiastically about the hundreds of thousands of poor Northeastern settlers who would follow the road builders in and build fertile new homesteads along the Trans-Amazon Highway.

It was one of the biggest gambles in South American history, and from its inception it had people from all over the world on edge -- biologists, anthropologists, businessmen, cattlemen. If the gamble paid off -- if Brazilians' every glowing prediction about Amazonia came true -- they were going to help reduce the debt with mineral and timbering wealth, and forge ahead with a completed national industrialization. Landless peasants would homestead fertile soil, enrich the nation's food supply and protect empty territory from foreign influence or occupation. Beef production would soar, and cattle for export and local consumption would roam lands that once held only trees.

If they lost -- if every scientific and sociological warning was ignored, and turned out to be correct -- they were going to destroy the greatest tropical rain forest on earth.

Forest burning would release large amounts of carbon dioxide into the air, raising the earth's atmosphere enough to melt the polar icecaps and flood the world's coastal cities. Whole species of mammal, insect, plant and bird life would be wiped out before scientists ever had a chance to study the potential effects of the damage.

Indigenous people would die by the hundreds as road builders brought disease and ranchers onto traditional Indian lands. Peasants would clear cropland on terrible jungle soils where nothing grew after the third year. All the structural problems of unbalanced land ownership -- hungry peasants, land invasions, people ripe for what Brazilian officials call subversive activity -- would spill out into the Amazon basin.

The geologist dos Santos has stayed in Amazonia to watch Brazil's 15-year foray into the jungle. How does he believe man has managed it so far?

"The worst way," dos Santos said without hesitation. "He's coming into Amazonia with the same greed, the same ambition he's carried everywhere else. I saw southern Amazonia 15 years ago and it was all forest. Now when I fly over it it's all burned. It should contain more food, of course, but it doesn't. It has less. Because it's been done in this prehistoric way -- man saying, 'This is mine' -- and not for the good of society."

Even in bare outline, what Brazilians have done with their jungle basin in 15 years is extraordinary. They have built a highway network that now covers 6,700 miles, half of them paved. Government officials list 350 large Amazonian cattle and agricultural projects that between them cover nearly 20 million acres.

Fourteen Amazonia airports have been built in the last five years. A massive seven-year mapping project, which is said to have cost the lives of two dozen people, laid out details of timber, geology and soil types for large hunks of Amazonia. For 1981 alone, a government official estimated that $107 million was spent on the three major development programs now at work in Amazonia.

But the gamble -- the great uncertainty about where all this will lead -- is still very much alive. Three centuries of conflicts, from the fight over land to the fight over multinationals to the struggle between frenzied development and ecological preservation, are playing themselves out in Brazilian Amazonia.

Ten thousand colonists a year now cross the border into Rondonia and in Ariquemes alone, there is a waiting list of nearly 1,000 colonists who signed up for homesteads. The city already has child prostitutes and poor parts of town and men who drink themselves into unconsciousness in the shade of the general store.

The territorial capital of Porto Velho briefly lost its water last October because erosion of newly denuded riverbanks filled the nearby river with silt. Airports in northern and eastern Amazonia have had to shut down temporarily because the smoke was so thick from jungle fires 100 miles away. Nationalists and government officials are at loggerheads over foreign companies' involvement in mining and agriculture projects.

Cattle ranchers have given up as inedible weeds took over what they thought would be fine pasture land. Measles, whooping cough epidemics and the sudden arrival of white road workers have killed or made beggars of hundreds of Indians.

Frustrated colonists have begun invading the property of other people, both Indian and white, and shoot-outs over land are not at all uncommon. On one long-invaded Rondonia reservation, Indians surrounded two young white men two months ago and shot them to death with wooden arrows.

"The first thing we feel when we come here is that they are destroying everything," Marcos Santilli, a Brazilian photographer now exploring Rondonia on a Guggenheim fellowship, said as he drank beer with friends one hot October night in Porto Velho. "But after some time you come back and talk with the people. And then you see that this is nothing more than a part of America, with all the history of the Americas happening at the same time.

"Here you have the most primitive people of the Americas, and you have the most sophisticated industries for minerals, or for wood. All the countries of American history are living together at the same time, at the same place."

This series is an outsider's look at Brazilian Amazonia -- the river dolphins and black piranhas, the heaved-together towns with black wiring still dangling looped from new telephone poles, the trees with roots that start above your head and braid down tangled into black swamp water.

Here are the red dirt highways where rolling trucks leave wakes of dust so thick that oncoming cars brake quickly and turn on their headlights, the hot sudden rain that smashes down into still afternoons and the muddy Indian village paths where buzzing fills the ears and every inhaled breath draws a noseful of insects.

"Sometimes in the midst of the stillness, a sudden yell or scream will startle one," wrote Henry Bates, the English naturalist who traveled Amazonia and described its wildlife from 1848 to 1859. "This comes from some defenseless fruit-eating animal, which is pounced upon by a tiger cat or stealthy boa constrictor. Morning and evening the howling monkeys make a most fearful and harrowing noise under which it is difficult to keep up one's buoyancy of spirit . . . . Sometimes a sound is heard like the clang of an iron bar against a hard, hollow tree, or a piercing cry rends the air. These are not repeated, and the succeeding silence tends to heighten the unpleasant impression which they make on the mind."

But what the stranger remembers most vividly now is the people wrestling with Amazonia itself and with the larger questions about the jungle's future -- the botanists, the anthropologists, the colonists, the Indian agents, the gold miners, the missionaries, the truck drivers, and the government officials who have both acknowledged past mistakes and fought off international criticism with the insistence that the "Amazon occupation," as they like to call it, is as inevitable and vital to Brazil as the opening of the West was to North America.

"We don't want to keep it as a place for our grandsons to come and say, 'This is a tropical rain forest,' " said Henrique Bergamin, the lively gray-haired chemist who directs the National Amazon Research Institute in Manaus. "We have to use it. We have half of Brazil here. We are not so rich as to keep this as a tourist place or something. We must use it. But to use it we must know how it works."