IN THE FAMILIAR paintings of New World explorers, we see them in silk sashes and plumed tricorns, planting their kings' billowing banners on some virgin landmark they've reached by the greatest courage and vision.

Thus is depicted the glorious culmination. The painter mercifully omits the tattered breeches, grease-and-blood-stained weskits, the boils, rashes, blisters, bruises, carbuncles, welts, dysentery, malaria, runny noses, snakebites, rotting gums, swollen tongues, and fetid, lousy linens one earns in the crossing of an untamed landscape. Such omissions make for a prettier picture, but perhaps we would appreciate the glory more if reminded of the myriad miseries those poor, slogging heroes had to endure to achieve it.

Herein lies one of the strengths of John Vernon's ingeniously crafted first novel, based on the last two journeys of Rene' Robert Cavalier, Sieur de La Salle, who in 1682 descended the Misssissippi River to its mouth and claimed its whole watershed for Louis XIV of France. The miseries are made real and pervasive.

And if La Salle thought he'd suffered plenty to get there, he hadn't seen nothin' yet. If he could have envisioned what lay five years ahead, he might have chosen to die right there on the delta while he was ahead.

To tell of this triumph and the 1687 disaster, author Vernon, a professor of English at the State University of New York at Binghamton, has chosen to revive an archaic form, the epistolary novel. The narrative unfolds through letters and two fictitious sets of journals, one by La Salle, the other by his map maker Pierre Goupil, an uneducated epileptic whom La Salle has taught to read and write and navigate.

This Goupil is a masterful creation, a loyal but reluctant Sancho to La Salle's Quixote. Goupil's journals delight with the terse eloquence of the justified complainer. He begins one day's entry: "a fine day everyone angry." This is after a canoe sinks because La Salle has, in a fit of pique the day before, kicked a hole in the bark of the canoe. Later, among Natchez Indians, Goupil complains:

"Their chief made a speech . . . Chuka who knows their tongue tried to translate Nicanape' translating Chuka, why can't everyone be born speaking French?"

A great part of the author's skill is in showing La Salle's obsessive personality through the writings of his semiliterate aide, while letting La Salle reveal his vainglorious notions of himself in his own journal. There is a contrast indeed. The purpose, though, is not to demean the hero, but to show little by little the flaws in his leadership which ultimately led to his murder and mutilation by mutinous followers.

La Salle was so self-confident that he believed he could return by sea and find the Mississippi's mouth even though Goupil had been unable to calculate its longitude. It was this enormous error of judgment that doomed La Salle's next expedition; unable to see the river's mouth from the Gulf of Mexico in 1686, he mistakenly landed and built a colony on what is now Matagorda Bay, Texas. Some historians have speculated that he deliberately went too far west because he had a secret plan to invade Mexico. Vernon chooses to show the great explorer simply and ironically lost, and while his outpost is decimated by shipwrecks, hunger, disease, Indian attacks, madness and hungry alligators, La Salle desperately thrashes through the inhospitable wilderness in all directions, trying to rediscover the lower Mississippi and the way back to Canada.

Finally, strung out to the end of their tether by hardships, hunger and hatred, a handful of his men murder him. When his loyal, bereaved cartographer Goupil survives this bloody coup and makes his tortuous way back to Canada, he finds himself accused of taking part in the murder -- and thus the irony is redoubled.

Such a hair-raising tale is its own reward. But here also are wondrous spectacles from both hemispheres: King Louis in his court dressed up like a savage. The Jesuits of La Salle's party and their eye-rolling despair over the natural ways of Indians. The slums of 17th-century Paris, where La Salle goes to recruit people for his colonizing expedition because he knows they are inured to suffering. Crude surgical treatments for epilepsy. La Salle's evil, mocking shrimp of a valet, Duhaut, whom Goupil calls the real murderer of the explorer. And there are vivid views of the primeval Mississippi wilderness, and its native inhabitants, who could be both delightful and deadly -- just like the Frenchmen themselves. Indeed, a key phrase of the tale is in the message left by some early deserters: Nous sommes tous sauvages.

AMONG the many journals of exploration acknowledged as sources by the author are those of Lewis and Clark, and it is plain that Vernon borrowed their literary styles almost intact for those of La Salle and Goupil respectively. La Salle's educated, subjective dissertations on himself and his mission read like those of Meriwether Lewis, even being in some stretches almost verbatim. Clark's short, telling sentence fragments and haphazard punctuation are echoed throughout Goupil's journal, again even to the use of identical phrases. Though this will nettle the admirers of Lewis and Clark, it does serve to keep the personalities of La Salle and Goupil consistent as we see them only through their own words.

La Salle's monomaniacal questing can be explained partly by his prospects for unlimited wealth; Louis XIV had granted him a monopoly on trade in the Mississippi Valley. And there are strong hints of madness. The Jesuit-trained La Salle believes the Jesuits are plotting against him, and in the midst of the most dire crises he sits down to tally up accounts of his debts in Canada. Goupil describes La Salle's mental state: "tomorrow he'll probably mope & pare his nails & wander off melancholy gnawing on his own forearm without anyone to whip . . . then return to beat those he imagines are his enemies and so on until none are left to beat."

In one appalling sequence, La Salle sulks among the sick and dying in his doomed Gulf fort and pens a play about Jason and the tarnished Golden Fleece, which he will make his wretches perform in a playhouse he has forced them to build with their last reserves of strength. The performance dissolves into a bedlam of lewdness, mass convulsions, religious frenzy and the stabbing death of the man playing Jason -- a horrifying scene which portends La Salle's inevitable assassination.

The murder itself adheres closely to the eyewitness account written by Father Douay, one of the Jesuits. Here it is told by Goupil.

Once again, literature finds the heart of darkness along a wilderness river, and Vernon has shown it for what it is: the sordid underside of glory.