The former England captain managed only 14 before a ball from Charl Willoughby stopped on him as he attempted to pull and he only succeeded in lobbing a gentle catch to Blackwell at mid-on.

This followed scores of 10, 0 and 19 and left Vaughan still searching for a return to form.

But unbeaten half centuries from Adam Lyth and Anthony McGrath helped Yorkshire to recover to 121-1 by the close.

Before Somerset resumed on 160-3, Yorkshire captain Darren Gough handed out first-team caps to leg-spinner Adil Rashid and left-hand batsman Andrew Gale, who was not playing because of an elbow injury.

It was not long before Yorkshire gained a wicket when at 167, Arul Suppiah edged a drive at Matthew Hoggard into wicketkeeper Gerard Brophy's gloves.

Then Tim Bresnan had James Hildreth edged to first slip where Jacques Rudolph held on to the catch after a juggling act.

There was a hint of good fortune about the dismissal of Craig Kieswetter which resulted in Rashid becoming the first Yorkshire bowler to reach 50 first-class wickets this season.

The batsman struck Richard Pyrah a blow on his shoulder at silly point with the ball bouncing off to reach Bresnan at short cover.

Peter Trego had just moved to 50 off 65 balls when he was bowled driving at Hoggard, who also trapped Steffan Jones lbw.

but there was no stopping Blackwell whose century came off 137 balls.

Bresnan made sure Blackwell ran out of partners by having Andy Caddick caught behind and then getting Willoughby floating a catch to Gough at mid-off.

Yorkshire experimented with a new opening batting combination in Vaughan and Lyth, but Vaughan soon perished.

Lyth, however, continued to show his potential in an unbroken second-wicket stand of 96 with McGrath.

The young left-hander eased his way to his half-century off 94 balls with nine boundaries and there was just time for McGrath to go to his 50 from 99 deliveries with eight boundaries.