It is not often that a minor seven-furlong race in early February decides anything conclusively about the Kentucky Derby. But that happened at Hialeah today.

Rockhill Native, the champion of his generation last season, had his Derby credentials torn to shreds in the Bahamas Stakes. While his fortunes were hitting rock bottom, a new star was rising.

Maryland-bred Irish Tower ran away with the race by 12 lengths, giving the most impressive performance by a 3-year-old this season. Rockhill Native could not keep up with him for more than a quarter-mile and finished third in the six-horse field.

The Bahamas was supposed to be a mere formality for the gelding who had never finished worse than second before today. Rockhill Native's opposition seemed moderate, and he was running at his optimal distance; he was the 3-to-10 favorite. The big questions about him were supposed to be raised when he attempted to go farther than 7/8 of a mile.

Instead, serious questions arose in the early stages of the Bahamas. Irish Tower popped out of the gate quickly from the outside, the speedster Native Command rushed up along the rail and Rockhill Native drove between them.

After a quarter-mile, the two leaders accelerated away from Rockhill Native, but the crowd of 11,949 saw that jockey John Oldham was just playing it cool. Only when Jeffrey Fell sent Irish Tower to a commanding lead on the turn did Oldham ask his mount for a serious response.

"When I asked him, he wasn't there," Oldham said. "He didn't fire. He didn't respond.He was dead."

By the time the field reached mid-stretch, the "bridge jumpers" who had bet more than $100,000 on Rockhill Native to show were wondering whether they should start looking for the nearest bridge. But as longshot Ray's Word rallied to get second place, Rockhill Native held on for third.

Trainer Herb Stevens was stunned. "He didn't run his race; why; I don't know," the Kentuckian said. But performances like Rockhill Native's are almost never flukes. They almost always hold portents for the future. Rockhill Native is surely finished for the time being, and he has about as much chance of winning the Kentucky Derby as the Ayatollah Khomeini does of capturing the New Hampshire primary.

But Irish Tower looked like a genuine star today. The 5-to-1 shot covered the seven furlongs in 1:22 3/5, excellent time on a track that has been deep and dull all season. Most speed handicappers would agree that no member of Irish Tower's generation has ever run so fast. Still, it is difficult to assess the colt.

All his life Irish Tower looked as if he were going to be a nice racehorse of the second rank. He won a couple of minor stakes in Maryland during the Summer, but when owner Hugh A. Grant died, he was put into a dispersal sale. Malcolm Winfield, a Florida sportsman, brought him for $375,000 and brought him south.

Here, too, Irish Tower looked like a nice colt but no world beater. In his last start, the Tropical Park Derby at Calder, he lost by 4 1/2 lengths to the unbeaten Superbity. Today he got a top-class rider for the first time, but trainer Stan Hough said, "I don't think the change in jockeys made the difference."

Hough had no idea what did make the difference. "When I saw him out front by that much," the trainer said, "I was speechless."

So was everybody else.