AN EXQUISITE ORDEAL. THE LAST FRONTIER. Cruising up the Amazon

THE Amazon Basin - equatorial host to primitive Indians, man-eating piranhas, and treacherous alligators - had always fascinated me as one of the few untamed areas left in the world. The Brazil that beckoned me was not sophisticated Rio or Sao Paulo or even the dark-continent allure of Bah'ia. It was the adventure of ``the last frontier,'' the dense jungles of the provinces of Par'a and Amazonas in the north.

My first view of the Amazon River, however, was not from the prow of a dugout canoe. It was from the balcony of my air-conditioned room, high up in the luxurious Bel'em Hilton. As I stepped out on the balcony, behind the high-rise condominiums of Bel'em, I could see the fabled, exotic Amazon, disappointingly mild and muddy, flowing lazily toward the sea.

Well, strictly speaking, it wasn't really the Amazon but one of its tributaries. A lush delta sat at the mouth of the river, where it emptied into the Atlantic Ocean and muddied it with Andes silt for almost 200 miles out.

From my vantage point, I could make out our boat, the ENASA (Empresa de Navega,cao da Amaz'onia SA) Line's Amazonas snug in its dock. I could also see the morning buses in the street taking people to work, and the palm-tree dotted square with its belle 'epoque opera house and Victorian iron bandstand.

This city of 1 million was hacked out of the jungle, mainly during the rubber prosperity at the turn of the century. I spent one day sightseeing in Bel'em, which was a stopover for United States troops during World War II on their way to fight in the North African war against Rommel.

A friend and I visited the famous Ver-O-Peso (Watch-the- weight) Market scattered along the waterfront. A series of beautiful fin de si`ecle iron structures that will probably soon be restored make this one of the world's most beautiful waterfronts. In one area, fishing boats brought in the day's catch. In other sections the stalls sold everything from exotic tropical fruit to monkey skins and stuffed piranha. The street adjoining the docks was crowded with shops selling cheap goods of every description, available at even cheaper prices after a few moments' bargaining.

Then we took a tour of the botanical and zoological gardens of the Goeldi Museum, which array examples of hundreds of varieties of Amazon flora and fauna. A weird and wonderful manatee, looking like E.T. grown old, swam in its own pond, surfacing now and then to observe visitors quizzically. There was no time to visit Marajo Island in the delta, where the pre-Columbian Marajoara civilization flourished. A craft shop in Bel'em, however, sells copies of the strange pottery, which their descendants still make.

Bel'em is quite close to the equator, and we were there in the dry season, when it only rains at 3 o'clock each afternoon. In the wet season, we were told, it rains several times each day. But the sun emerges immediately after each downpour and dries everything off in a matter of minutes.

We stopped in at the offices of ENASA, with which we had booked our six-day cruise up the Amazon from Bel'em to Manaus on the catamaran Amazonas. After having experienced some of the casual uncertainties of reservations in the south, we were relieved to find that we were on the manifest and scheduled to board the next day.

Despite warnings from travel agents and Brazilian friends that a Brazilian cruise ship would come sans such luxuries as air conditioning, even though it was promised, we were pleased to find after boarding the ship the next day that our tight but comfortable two-bunk cabin was cool and pleasant. So were the public lounge and the dining room. All cabins had private bathrooms.

The catamaran vessel, exceptionally smooth-riding and able to carry 134 passengers, was actually carrying 104. Later we discovered that the passengers had paid a variety of fares, ranging from about $400 to $1,200 per person, for comparable accommodations. We had paid $480 per person for the six-day voyage and had refused to double the ante, as was unexpectedly demanded just before sailing. When we threatened to cancel, the original price held.

We were the only US citizens on board. There was one Canadian and a party of 36 Uruguayan architects and friends. All the rest were Brazilians, who seemed content during the six-day voyage to engage almost nonstop in the only planned activity aboard: bingo, interrupted three times a day for meals.

The meals were served buffet-style, which meant queuing up in Army chow-line fashion. The food was presented in huge quantities, usually covered with a manioc sauce that resembled the ``oyster'' sauce in inexpensive Chinese restaurants in New York. The best meal turned out to be breakfast, when fruit juices, fresh fruit, cheese, and ham were available.

On the first day, settlers living on the delta islands rowed out in canoes to greet the vessel. We were provided with plastic bags filled with candy and toys to throw to them. For the rest of the voyage we passed homesteaders every few miles, eking out lonely existences on the banks.

The relentless jungle scenery became boring after a few hours, though one thatched-roof house had a fake wooden TV antenna attached to the roof - possibly a joke or a symbol of relative prosperity in an area that seemed barely above the subsistence level, without electricity or roads. The river apparently provides the only means of transportation, and we constantly passed smaller vessels loaded down with passengers whose hammocks were slung on deck.

When the ship docked at the halfway point, Santorem, ENASA had planned a ``city tour,'' which started with visits to a hammock factory and a souvenir shop.

Then we were herded in to the Santorem Yate (Yacht) club, a rather primitive disco on the river's edge where the passengers, especially the determinedly festive Uruguayans, danced to loud samba records. That proved to be the entertainment highlight of the voyage. Meantime, the Amazon flowed by - brown, brackish, and boring - from the deck of the ship.

What mysteries would the jungle hold, we wondered. Since we had booked into a jungle lodge after the arrival in Manaus, we hoped we would discover the real Amazon there. (See facing page for report on Manaus.)

On the top deck there was a small swimming pool with a few lounge chairs; otherwise there were no comfortable deck chairs, only upright seats fine for reading but unpleasant for relaxing. The prospect of luxury cruising with afternoon bouillon, midnight snacks, and constant service was obviously beyond the means of ENASA. But it did get us to Manaus on time.

We could have flown from Bel'em to Manaus, 1,700 kilometers (1,050 miles), in less than two hours. But instead, for six days we endured a ``luxury cruise, Brazilian style.'' That is very different, we discovered, from a luxury cruise elsewhere in the world. If you go

The Bel'em Hilton, which a few months ago charged $60 a night for a double, is now charging $120. Check with Hilton International before booking.

The cruise can be booked directly through ENASA (Av. Presidente Vargas 41, Bel'em, Brazil); through a travel agency such as Abreu Tours either in Rio or New York (60 E. 42nd St., ZIP code 10165, tel. [212] 661-0555); or through Amazon Explorers, PO Box 815, Parlin, NJ 08859, tel. (201) 721-2929.

Amazon Explorers offers an 11-day land and ship tour of Bel'em, the Amazon, and Manaus for about $1,400 per person. ENASA offers just the ship for about $650. ENASA also owns a fleet of nonluxury boats that ply the Amazon, and for about $50 it is possible to book deck passage on a river boat, if you are willing to sleep in a hammock.