The $346,800 Jockey Club of the season, drew a solid field of 13 horses today at Belmont Park.

Affirmed and Alydar, the best 2-year-old in the land, headed an excellent field in the $134,000 Champagne Mile.

The $529,500 offered by the New York Racing Association on the nine-race card represented the richest program in the history of American racing. Steve Cauthen, the 17-year-old riding sensation, had a chance to pass the $5,000,000 earnings mark.

The weather cooperated beautifully. It was a cirsp, clear autumn afternoon.

Yet only 23,785 fans turned out. Admittedly, the Yankees were in the World Series on television from Los Angeles and Army played Notre Dame in Giants Stadium, across the Hudson. But 23,795?

For the record, an 15-to-1 chance named On The Sly won the Gold Cup. Alydar defeated Affirmed and Cauthen in the Champagne.

On The Sly, a plodder that had won $173,275 this season by beating about the bushes, never had impressed New Yorkers, who made Crystal water 9 to 6 and Great Contractor 2 to 1 today. So On The Sly proceeded to take the Gold Cup by 5 1/2 lengths over Great Contractor, which barely edged Cox's Ridge for second money.

Gregg McCarron, a Maryland regular, enjoyed his greatest day as a jockey aboard On The Sly. McCarron guided the winner to the lead after 1 1/4 miles in 2:04 en route to a 2:08 1/3 clocking over the 1 1/2 mile run in muddy going. The victory was worth $208,080, leaving trainer Mel Gross slightly dumfounded for the national TV interview.

"Mel's been dying for a mile and a half race for this horse," McCarron supplied. "We were in front by four or five in midstretch, but I kept after him. After all, how often do you get a shot at a $300,000 race?"

Crystal Water, the favorite, could finish no better than fifth after taking the lead briefly near the mile pole.

On The Sly had won only once since April, before today. He is type of runner that often gets a part of the purse, without winning, although in receipt of several pounds from more highly fancied horses.

Competing under weight-for-age conditions today, the 4-year-old. On The Sly and all the other older runners carried 126 pounds apiece. The 3-year-olds, such as Cox's Ridge, carried 121 each.

Laurel Race Course might well have been the big winner of the Champagne Stakes.

Affirmed, with three decisions over Alydar in four meetings was favored at 6 to 5. The Calumet Farm colt was 3 to 2. A victory by Affirmed would have wrapped up the 2-year-old championship, but Alydar was very convincing in scoring by 1 1/4 lengths over his arch rival. The Laurel Futurity at 1 1-16 miles Oct. 29 is likely to settle their argument.

A change in jockeys was important today. Jorge Velasquez replaced Eddie Maple on Alydar, trainer John Veitch noting. "When you get beat a nose like we did in the Futurity, you can get desperate."

Velaquez guided Alydar from well off the pace to collar Affirmed shortly inside the eight pole. Alydar drew off to regular smartly as the favored took second place by the identical margin over Darby Creek Road.

Alydar returned $5 straingt after completing the mile in 1:36 3-5. First money was $80.400.

"We'll definitely supplement to the Laurel race," Veitch said. "I hope Affirmed will be there too."

Trainer Laz Barrera indicated before the Champagne that Affirmed would not be going to Laurel. Today's result probably will prompt him to change his mind - if he wants to be sure of the Harbor View Farm colt gaining an Eclipse Award.