I do not how to do citations, but All Music Guide calls it "Hard Rock" and the reviews by Dave Ling and Pete Freensta, while not calling it hard rock, say that it has a heavy ("very heavy in places" -Dave Ling) sound to it, returning to their "proggy roots" (All of these early albums, exempt of "Hemispheres" and "A Farewell to Kings" have progressive rock and hard rock on their gerne lists).  Sufficient?