Name your favorite rock band. If you said The Dave Matthews Band, Sheryl Crow or Coldplay, go back and start again. They may no longer qualify. Rock'n' roll is no longer some nebulous, all-encompassing catch-phrase category. It has been clearly defined by Marah. The Philadelphia-based band opened their current tour with a show at the Iron Horse Music Hall Tuesday, ripping through a 90 minute set of rock that left half the 100 or so fans screaming for more, and the other half shaking their heads in disbelief. Every decade or so a band will come around that breathes life into the original vision of rock. Marah has re-lit the torch and carries it well. Marah is rock'n' roll not just because of what it sounds like, but what it is; a pair of brothers, the Bielankos, who used music to kick and scratch their way out of the garage and on to Iron Horse-like stages around the country.