Cowan, taking full advantage of the humid, overcast conditions, dismissed the top five in the order after Glamorgan had won the toss for the first time in a Championship match since the victory over Gloucestershire in mid-May.
The batsmen had to work hard for their runs and Cowan, Ronnie Irani and the impressive Ricky Anderson, who finished with 3 for 35, found extravagant movement both through the air and off the pitch.
Essex lost Paul Grayson, bowled by a shooter for 27, and Nasser Hussain, hooking to Evans on the square leg boundary, but Paul Prichard and Law re-established control with an unbroken stand of 71.
Robin Weston was the day's top scorer closing unbeaten on 145 for Derbyshire - who finished on 314 for 6 against Somerset at Derby. There were half- centuries for Karl Krikken (56) and Ben Spendlove (51).
It was a marathon day for Andrew Caddick, who bowled 27 overs and finished with figures of 4 for 59.
In the Vodafone Series, the New Zealanders' tail-enders enjoyed the limelight as Dion Nash made an unbeaten 106 and Daniel Vettori 50 out of a total of 274 for 7 against Hampshire at Southampton.
Muttiah Muralitharan sat out the visit of the Sri Lanka A side to Old Trafford. Instead it was Ian Austin who provided Lancashire's greatest threat, taking 6 for 43 as the tourists were dismissed for 277. Lancashire, with stand-in captain Mike Atherton back in the pavilion, were 53 for 3 at stumps.