In 1848 the United States signed a treaty with Mexico, realizing its Manifest Destiny and acquiring a dubious chunk of real estate extending from the Texas border to the Pacific Ocean. California became a sudden and irremediable wrinkle in the American psyche a few months later, when news of the discovery of gold leaked out. Americans who had arrived in California on the eve of the Gold Rush found people who "sleep and smoke and hum," a fair approximation of West Coast mores today. Those natives were soon overcome by thousands of prospectors from the eastern states, Latin America and Europe drawn by newspaper reports of easy pickings among the streams of the Sierra Nevada.

California from the beginning was synonymous not just with quick wealth, but also with the chance to wipe out burdensome mortgages, to buy land and businesses at home, to acquire independence and even happiness, a special California commodity that still influences our politics and our fantasies. An "extraordinary mania" gripped the country, as the New York Herald put it, affecting the most stable elements of American society. Family men as well as adventurers formed companies, borrowed money for supplies, kissed wives and lovers and hi-hoed the open trail. For most of them the trip west was a far more memorable experience than the time spent in El Dorado.

The author of "The World Rushed In" has taken a disparate, near-mythic American experience and given it coherence and spontaneity, a real accomplishment when one considers the glut of available material. It seems unlikely that anyone will write a more comprehensive book about the Gold Rush. Holliday has drawn on hundreds of diaries and letters to recreate imaginatively that experience, concentrating on the career of a remarkably literate farmer from Youngstown, N.Y., named William Swain. He was one of 30,000 prospectors who used the South Pass trail between 1849 and 1851, all of them suffering at some time from bad water, dysentery, torrential rain, hailstones the size of goose eggs, opportuning or threatening Indians, and a diet of salt pork and flour varied with "bush fish" -- rattlesnakes -- prairie dogs, marmots and an occasional buffalo steak.

They left behind uncertainty and some bitterness. An affecting sub-theme of the book is the reevaluation by the stay-at-homes of the departed, and vice versa. "Hitherto I was a stranger to my attachment," wrote Swain's wife, Sabrina. Swain came to a new appreciation of the stationary life, as well as the skills of sewing and cooking. "I have always been inclined," he admits to Sabrina, "to deride the vocation of ladies until now."

Other niceties faded just as surely. An increasing callousness was shown toward men dying of cholera, the main fear of the gold seekers. Disease further slowed the arduous progress already limited to the plodding gait of oxen, while men were busy gathering nuggets beyond the mountains. A diarist looked into a wagon and saw a man "still alive and smothered with dust and blankets. They buried him in a most inhuman manner, perhaps alive." The graves were shallow.

Roadside litter was a problem from the beginning. Spent equipment and trash littered the campsite; the trail abounded with shovels and other tools tossed out to lighten the loads. Graffiti was another urge of the first transcontinental tourists. They wrote and carved on every available surface, including tombstones, bleached buffalo bones and famous landmarks.

Men forced to live together in deprivation quarreled and fought; standards were forgotten amid hardship and anxiety. "We have seen a man eating his lunch gravely sitting on the carcass of a dead horse." "We see some of the most grotesque figures, living caricatures of the human species . . . dressed in dusty, ragged clothes." Those who made it over the Sierra Nevada suffered from malnutrition, scurvy and broken spirits. Men "mounted on mules . . . had to be lifted off their animals."

All the suffering went unrewarded, except in a few cases. Easy diggings were played out before Swain and the mass of men arrived. That meant staking remote claims or buying interest in others. Eventually streams and rivers had to be diverted to get at the deep veins of collected gold -- a difficult and expensive process that usually proved unproductive. In the meantime there was food to buy at wildly inflated prices, and the lures of gambling, whiskey and circuit-riding whores.

The dream of California was converted into the reality of returning home with a mere pocketful of gold dust, or nothing at all. The way lay through the isthmus of Panama, and more encounters with violence, natural disaster and disease. Swain was one of the lucky ones: He arrived back in New York sick, but salvageable. He had a few hundred dollars left from his gleanings, and a working farm and family in place.

Swain and thousands like him had "seen the elephant" -- a farmers' expression for undergoing an exotic and improbable experience. The irony is that none of them saw the real gold in the West, the great agricultural potential of the Sacramento Valley and the commercial future of the Pacific Coast. Swain lived to regale his grandchildren with tales of a fabulous land on the far side of America; for their grandchildren California would seem quite different, yet curiously the same.