From five thousand feet, the Amazon jungle below is a giant checkerboard of brilliant green and dull gray-brown. Patches of luxuriant tropical growth alternate with swaths miles square in which all vegetation has been cleared and burned and nothing remains except ashes, tree stumps and columns of smoke curling to the sky.

"There are days when it's much, much worse than this," pilot Sidney Salles tells his passenger. "Back at the height of the dry season in August, the smoke from the forests being burned was so thick that you couldn't even see past the nose of the plane."

From one end of the Brazilian Amazon to the other, the sight is much the same. The world's largest tropical rain forest is being devastated to make way for ranches, farms, mines, roads and settlements -- activities that many scientists and agronomists argue are turning the Brazilian Amazon -- an area two-thirds the size of the continental United States -- into a vast wasteland.

"The destruction gets cheaper and more efficient every year," says Harry Knowles, a former U.N. ecologist who has spent 22 years in the Amazon studying forest conditions. "If deforestation continues at its present rate, the Brazilians could very well end up creating another Sahara."

Until recently, e extent of such damage could only be guessed. But based on analysis of 32 photographs taken from a Landsat satellite, Brazil's National Space Research Institute estimated earlier last month that as much as one-tenth of the Brazilian Amazon forest has now been razed -- an area bigger than the state of Texas.

The systematic leveling of the Amazon may have only begun. The Brazilian government admitted recently that it is studying a plan to allow Brazilian an multinational timber companies to sign "risk contracts" for the large-scale harvesting of wood in 12 selected areas of the Amazon.

According to Hugo de Almeida, head of the Superintendency for the Development of the Amazon (SUDAM), approximately 100 million acres have been earmarked for potential timber exploitation. The program is being touted in some government circles as a way for Brazil to ease the burden of its soaring foreign debt, which has now reached $40 billion.

"The forests that today supply 85 percent of the world market will be exhausted around the year 2000," said a spokesman for another government agency, the Brazilian Institute of Forestry Development. "International demand is going to provoke a rush to the Amazon, and it is exactly through these risk contracts that we plan to regulate that rush."

Scientists argue, however, that timber felling on the scale proposed would lead almost inevitably to the creation of an Amazon desert. The same damage results, they add, from the slash-and-burn agricultural techniques used by the ranchers and peasant farmers who have flocked to the Amazon sice the opening of the TransAmazon Highway this decade.

The crux of the problem is that the Amazon's appearance of eternal fertility masks one of the world's most fragile ecological systems. In the words of American scientist Betty Meggers, the Amazon is a "counterfeit paradise" -- a jungle whose lushness derives not from its soil base but from the continuous recycling of nutrients through dense forest cover.

Indeed, studies have shown that most of the soil of the Amazon is thin, infertile and highly acidic in content. Recent air photo and radar surveys undertaken by the Brazilian government have indicated that only 2 percent of the Amazon is suited for agriculture.

In spite of these findings, the deforestation of the Amazon continues. Here in the remote but booming territory of Rondonia, a chunk of the western Amazon the size of West Germany, thousands and thousands of acres are being stripped of vegetation and turned into cattle ranches an cocoa plantations.

Elsewhere in the Amazon, Brazilian officials speak with enthusiasm of plans to raise rice, beans, corn and cattle and settle thousands of poor peasant families from other regions. More than 7.5 million acres have been cleared for Amazon highway projects, and the government is financing scores of mineral exploration programs.

In most areas of the Amazon, trees are destroyed by burning or are uprooted with correntoes -- giant chains 100 yards long and weighing up to 10 tons, attached on either end to tractors. And, according to Brazilian press reports, the highly toxic Agent Orange is also being used to clear land for cultivation.

Agent Orange is a chemical defoliant widely usd by American military forces during the war in Vietnam. Its application in the United States is now restricted as a result of medical studies linking it to birth defects, miscarriages, liver cancer, leukemia and nerve damage.

"You can find Agent Orange of the shelves of farm supply stores in Manaus," capital of the state of Amazonas, said Orlando Valverde, an agricultural geographer at the Brazilian Institute of Geography and Statistics who recently completed a three-month study o farming methods in the Amazon. "All you have to do is walk in the door and ask for it."

Once cleared and subjected to intensive farming methods, the thin Amazon topsoil is quickly leached of valuable nutrients. Combined with the widespread deforestation that has taken place in the region over the last decade, the result has been the rupture of a delicate ecological cycle thousands of years old.

"When you can cut or burn huge areas, the remaining forest can no longer hold the rainfall and act as a sort of sponge, letting the water run out slowly," explains ecologist Knowles. "The absence of forest cover allows sheet erosion, which is followed by gully erosion, and before you know it, the land is no good for anything."

This has led in some areas to dramatic changes in the environment. The city of Maraba, located near the confluence of the Tocantins and Araguaia rivers, is precisely the area satellite photos have indicated as most seriously devastate. The city is now flooded annually and ecologists blame silted-up rivers and lack of forest cover.

In other regions of intense cultivation, the jungle-to-desert process has already taken place. The largest and best-known example is the Bragantina zone around the city of Braganca at the eastern end of the Amazon region. The Maryland-sized area of tropical forest has been transformed into what one Brazilian scientist has called "a ghost landscape."

Settled by European and Brazilian immigrant farmers around the turn of the century, the Bragantina area was at first blessed with high yields. By the 1940s, however, the soil had been exhausted and crops were replaced by coarse grasses and, eventually, by huge expanses of bare soil and rock unable to hold water.

In te late 1940s, some of the Bragantina farmers were resettled by the Brazilian government at a colonization site along the Maderia River north of here. There, in the far western Amazon near the Bolivian border, their unhappy experience repeated itself in an even shorter time.

Once again, the results from the first three years were promising. But within five years the once-dense jungle had been turned into what one study called "virtual pavements of rock," and today the colony is nothing more than a huge expanse of exposed and exhausted soil on which a few poor and desperate settlers eke out a meager living.

"The Bragantina experience holds lessons for all of the Amazon," says Knowles. "When the forest has been destroyed and the soil used up, there's nothing that can be done except to pack up and go away."