The Deal: All-instrumental act from Portland delivers another exotic gem.

The Good: The best instrumental music says more than traditional songs because the listener's imagination gets to do all the heavy narrative lifting. When I heard the Grails' evocative eight-minute epic, "Silk Rd," I found myself neck-deep in images of Alexander the Great, Marco Polo and Genghis Khan's hordes inspired by the song's dramatic arcs and thundering herd crescendos. Finally, a soundtrack for that Intro to Humanities freshman course!

Grails counts among its members drummer Emil Amos, the twisted mind behind the deadpan darkness of the Holy Sons, and he is the engine driving most of these eight cinematic cuts. But the other three Grails' members -- including Zak Riles, M. Ward's guitarist -- provide the Middle Eastern textures, ranging from acoustic guitar drones and windswept synth washes to Oud-like runs and sitar-flavored psychedelic freak-outs. Impurities may not have the raw improv feel of earlier Grails' records, but the raga-like processions and dynamic ebb-and-flow maintains that marvelous collaborative essence that only comes from years spent pushing at the boundaries of traditional song structures.

On your handy post-rock slide rule, Impurities would be located somewhere between Town & Country's acoustic drone, Popol Vuh's world-beat exotica, Ummagumma-era acid flashbacks and Mogwai's slow-build-and-burn.

The Bad: Only if you need someone to sing you the story.

The Verdict: It's hard to throw a mic-stand without hitting an all-instrumental rock outfit these days, but songs this good are the best reason to let the music do all the talking.