This excerpt is taken from a work that is much longer and is scored for a colorful combination of instruments.  One level of "activity" comes from the continuous patterns from the mallet instruments.  The other level of activity is very slow by comparison.  The tonal center is established by the repetition of motives on the same pitches.  The long, repetitive, almost hypnotic music has no sudden changes in dynamics or texture.  Reich's study of African drumming and Balinese gamelan music influenced his repetitive writing style.  The long, sustained notes give his music a weak sense of tonality; however, it is difficult to perceive the chords as a traditional harmonic progression.  Tonal stability is established merely by repetition.