PAUL MELLON was not there when the Yale Center for British Art and British Studies - his radiant gift to Yale and to all of us - opened to the public Tuesday in New Haven. At that culminating moment he and Christmas Goose, his horse, were setting off together for a 100-mile trail ride through the mountains of Virginia.

Paul Mellon will be 70 in June. The Center's every detail - the subjects of its pictures, the proportions of its galleries, the wholeness of its spirit - reflect the life he's led, a life of country houses, country sport, and deep esthetic pleasure.

He conceived the institution - part library, part classroom, part wonderous picture gallery, it is dedicated equally to scholarship and beauty. He paid for the building: project cost, $12.5 million. He bought the British objects in it, the 16,000 rare, illustrated books, the 1,800 paintings, the 7,000 drawings, the 5,000 prints, the 10,000 reference volumes, the 60,000 photographs, the marble statues and oak chairs.

He and Christmas Goose rode 40 miles Tuesday, 40 miles Wednesday, and 20 miles Thursday. It is an annual event, the 100-mile trail ride. Speed is not important, since the victor is determined by the condition of his mount. Mellon won the championship in 1959, on Silversmith, and in 1967, on Warlock. Last week he won again. "It is," says the philanthropist, "exhausting for the horse."

Mellon has a famous fortune, as many people know. His father, Andrew Mellon, served as Secretary of the Treasury and as ambassador to the Court of St. James. Andrew Mellon gave the nation the National Gallery of Art. Paul Mellon is its president. He first held the Gallery job in 1928-39, before the building opened, and took it once again in 1964. Mellon raises racehorses. That, too, is common knowledge.

What was not widely realized before the Yale Center opened, but is apparent now, is that as an art collector - collector, not investor - Paul Mellon has few peers.

What is even less well known is that he was analyzed by C.G. Jung, the philosopher-physician; that he underwrote, in 1950, the American publication of one of hippydom's, and China's, most beloved books - the I Ching or Book of Changes (in the Richard Wilhelm translation); that he has been, at various times, a soldier and a scholar, and that he is related, although very distantly, and only by the recent marriage of a former son-in-law, to none other than Liz Taylor.

Those who know the Mellon name know the Mellon fortune is very, very big, so big the figures cited tend to be crude guesses. The Mellon family at one time owned the biggest bank in Pittsburgh, one-third of Gulf Oil, almost one-third of Alcoa, as well as blocks of stock in General Motors, Pittsburgh Plate Glass and other corporations.

Nine years ago, when a cup of coffee cost a dime, and only one American in 50 enjoyed an annual income of more than $25,000, Fortune magazine reported that three of the Mellons - Paul, his sister, Ailsa Mellon Bruce, and their cousin, Richard King Mellon - were together worth between $1.5 billion and $3 billion.

Having glimpsed the Yale Center, the editors of Time last week estimated that its contents were worth "close to $200 million." In Washington it is said that Mellon, his late sister, and their family's foundations, have given another $100 million to pay for the construction of a new East Building for the National Gallery of Art. The dollar figures cited all are nice and round.

Those who read the sports pages know Paul Mellon raises racehorses on his 4,000-acre Upperville estate. His thoroughbred, Arts and Letters, was named horse of the year in the 1969; his Fort Marcy won the same distinction in 1970, and after his Mill Reef - "the best horse I've ever owned" - took both the Epsom Derby and Prix De LArc de Triomphe, Europe's richest race, in 1971. Mellon was selected horse owner of the year.

If great wealth is a blessing, it may also be a burden. In America, and not only in America, but even in Paul Mellon's beloved Britain, the very rich are often viewed with jealousy and scorn. Journalist Henry Fairlie, who writes for London's Sunday Times and occasionally for this paper, did not say "the filthy rich," though that's what he implied, when recently he wrote, "The Cezannes and Matisses hang on their walls like certificates of stocks and bonds. For the most part, the very rich in the United States use their fortunes only in order to maintain and increase their fortunes . . . they may even be said to have forfeited one of the few justifications of great wealth, that it is able and willing to waste money. The very rich in America are little more than their own safety-deposit boxes."

That is not the impression given by Paul Mellon. Paul Mellon is a teacher.

What moves the viewer most at his Center in New Haven is not the lightfilled space, so classically proportioned by the late Louis Kahn, nor the brilliant works on view by painters famous and unknown. What is moving is the harmony. Every aspect of the Center, the reticence of its architecture, the completeness of its library, its many views of England's green and pleasant landscape, the spaniels and proud horses that populate its always gentle pictures - everything one sees seems to have grown naturally from Mellon's happy life. He might have bought an airline. Instead he's spent his time, his cash and connoisseurship conveying to the public that which he most loves.

Images of Mellon listening to Jung, or spending millions on old pictures, or up on Christmas Goose, riding through the mountains, are not in contradiction. They are part of the same whole.

Given his great wealth and the example of his father, he might have started buying pictures early. But he didn't. The bulk of his collection has been purchased since 1961.

"I don't believe many motives in life are clearcut or self-evident," says Mellon. "Collecting especially is such a matter of time and chance - intellectual bent, individual temperament, personal taste, available resources, changing fashion - and my own motives as a collector seem to myself extremely mixed." They are clearer now to others. What promoted him was love, love of horses, love of learning, love of landscape and of England. All these loves are manifest in the collection he has gathered. They are also interwoven, strung behind him like a tail, in the happy life he's led.

"My mother was English. From 1907 until 1914, from my 1st year to my 7th, my parents spent almost every summer in England," Mellon has recalled. "I suppose it was in those summers that I first developed a taste for the English countryside, for English houses, English rivers, English parks, English skies, English clouds - and let's not forget English trains . . . From those distant summers I remember huge dark trees in rolling parks, herds of small friendly deer, flotillas of white swans on the Thames, troops of gray horses. . . and always behind them and behind everything the grass was green, green, green.

"Somehow at this great distance it all melts into a sunny and imperturbable English summer landscape. There seemed to be a tranquility in those days that has never again been found and a quietness as detached from life as the memory itself."

That memory of quietness, of England's green tranquility, has never left Paul Mellon. He strengthened it at Yale, where, in 1929, he won the first McLaughlin Prize in English, and "confirmed my predilection for things English and things old." He deepened it at Cambridge, where, already "a galloping Anglophile," (he puts stress on galloping), he says "I r-o-d-e constantly, I r-o-w-e-d intermittently, I r-e-a-d a little." It was while at Cambridge that Paul Mellon purchased his first rare and costly book - Strutt's "Sports and Pastimes of the People of England." His collection had begun. He did not know it at the time.

His first wife, Mary, died, following a heart attack, in 1946. When she married Mellon in 1935, she suffered from an allergy - to horses - which she thought psychosomatic. Analysis was suggested, so the Mellons went to Zurich to consult Dr. Jung. They stayed more than a year. John Walker, a former National Gallery director, a fellow Pittsburgher, and a childhood friend of Mellon's, says that after seeing Dr. Jung, Mellon "became one of the best balanced and, as far as an outsider can ever know, one of the happiest of human beings."

Did it change his life. Did the treatment stick? Mellon spreads his hands. "How does one know?" he asks.

After World War II began, Mellon, not surprisingly, joined the U.S. cavalry. Enlisting as a private, he rose to first lieutenant, then transferred to the OSS and rose again to major. When peace returned, Paul Mellon enjoyed it in Virginia. By now he had begun to buy "in a desultory way" English sporting art.

He continued studying. He spent a year at St. John's College in Annapolis (and later found that school some $9 million). He underwrote the Bollingen Series of the fine books (among them the I Ching ). In 1961, the Virginia Museum in Richmond asked Mellon, a trustee, to pick the English pictures for a temporary loan show called "Sport and the Horse."

It was that borrowed exhibition that made Mellon a collector. In the next two years, with the help of Basil Taylor, the English art historian, Mellon purchased for himself more than 400 pictures. He had just begun.

"Dealers were spellbound by the magnitude of orders they received," writes Walker. "Photographs, transparencies, drawings, paintings, arrived in America by the plane-load. The available supply of British art, accumulated over a long period of disregard, was so great that prices for a surprising time remained low." Mellon, in effect, cornered English art.

The market then was ruled by "Matisses and Cezannes," by Italian and Dutch pictures, and by post-war U.S. art. "English art," says Mellon, "seemed to me long neglected or even abandoned, not only in this country, but also in its homeland." Only a few British painters - Turner, Constable, Hogarth, Blake - were then thought worth collecting. Mellon bought their works in depth - 120 Constables, 42 Hogarths, 70 Turners - but he also collected Zoffany, Bonington, Marshall, Moreland, Mortimer, Palmer, Ward, Wheatley, Harlow, Havell and scores of other artists whose names were unfamiliar to most American collectors. Unlike, say, his father, he did not only seek blockbuster masterworks. He bought pictures great and small.

His collection portrays Britain, its horses, coaches, theaters, spas, musicians, country houses, country gentry, buildings, boats, and birds, its landscape and its skies. He bought sketchbooks, maps, cartoons.

Great collections teach Duncan Phillips' gallery shows the possibilities of color and the roots of modern art. Joseph Hirshhorn's surveys modern sculpture and shows us what he saw in New York's commercial galleries. Mellon's Center shows us England's gentle past.

Though most of his British art has been given, or pledged, to Yale, Mellon still collects. He has another fine collection, assembled with his wife. Bunny, of French impressionist and post-impressionist art, including a superb group of waxes by Degas. He also has some pictures by Mark Rothko.

He is not through buying, nor is he through giving. He has reserved some English pictures for the Virginia Museum, and for the Tate Gallery in London, and he has set aside a number of his finest English paintings - a Fuseli, a Stubbs (George Stubbs, the 18th-century horse painter, is Mellon's favorite artist) - for the National Gallery here.

The Yale Center for British Air and British Studies is regarded, rightly, as a triumph for Paul Mellon. He has one more in store. When he describes himself in "Who's Who" as a "museum executive," he is referring to his post at the National Gallery of Art. More than any other man, he is building its new building. He chose I.M. Pei as architect (Yale selected Kahn). The new museum on the Mall also will house a library and a study center.It is scheduled to open in late spring, 1978.