Saturday was an action-packed day at the 2015 NFL Scouting Combine, with quarterbacks, receivers and running backs participating in the on-field portion of the week. Here are four observations from all the action.

1. Top QBs looked goodThe top two QBs in this year's class both had good days. Marcus Mariota clocked a 4.52 in the 40-yard dash, tops among all players at his position, while Jameis Winston dazzled in the throwing drills.

Former Vikings Offensive Coordinator and Super Bowl-winning Baltimore Ravens Head Coach Brian Billick was impressed with Winston.

2. Kevin White strengthened his case to be class' top WRThere aren't many holes to shoot in former West Virginia WR Kevin White's game. He has production, he has size, he exudes confidence without seeming too brash. But the one question some scouts raise about White's prospects to be the top-rated receiver in this year's class is speed. White answered that question on Saturday. He ran a scintillating 4.35 in the 40-yard dash, good for third-fastest and bettering the times of both Amari Cooper and DeVante Parker, a pair of receivers who could arguably be rated ahead of White. Now, though, it's tough to put anyone ahead of White in this year's WR class.

3. But Cooper and Parker won't go away quietlyWhite may have impressed with his 40-yard dash time, but it's not as if the other top-rated prospects withered away. Cooper clocked a 4.42 and Parker came in at 4.45, both well within the "he's fast" range. One can't help but to come away from the Combine thinking this year's receiver class features three impressive prospects and arguments can be made for each of them that they are the cream of the crop.

4. Gophers' David Cobb comes up with apparent injury in 40-yard dashThe RB class is another deep one in this year's draft, but one of the better ones and one close to the hearts of many Minnesotans, David Cobb, will have his Combine experience cut short. David Cobb injured his leg during his 40-yard dash and will likely not participate in any more drills.

The good news is NFL Media analyst Mike Mayock says not to fret.

"I feel badly for him," Mayock said. "He's not going to run 4.3. I'm sure he'll be able to run in his pro day, so it won't hurt him."