Unlike Stax, and other venerable labels with star-studded rosters whose catalogs have been excavated recently, Excello Records boasts no names more familiar than Slim Harpo or Lightnin’ Slim. But what these two collections of electric blues, R&B, and rockabilly from 1954 to 1966 can offer is a deep, satisfying dip into the muddy Southern waters of rock & roll’s birth. Most of the Louisiana natives (including both blues-busting Slims) on Vol. 1 (Sound of the Swamp) serve up a gritty, impolite guitar-based response to the smooth New Orleans piano pop of folks like Fats Domino. Vol. 2 (Southern Rhythm & Rock) tracks the diverse stream of cosmopolitan R&B and vocal-group soul that issued from Excello’s Nashville branch, music distinguished from Vol. 1 by relatively polished performances and ingenious songwriting. Vol. 2 is the more immediate pleasure, but both discs promise an ideal antidote for those who find today’s chart fare heartless and contrived. A