This modulation is remarkable not only in itself, but in the context of the Kyrie’s larger tonal structure. The piece’s overall key is F-sharp minor. The cascade we’ve been considering begins elsewhere— A major— but A major is as close to F-sharp minor as a key can be (the two keys have all their notes in common);

a modulation between them can be effected as quickly and directly as this:

Now look at the harmonic progression by which Bach’s cascade covers this same, tiny tonal distance:

Mark Twain once watched an ant traverse proximate points in the dirt via a round trip to the top of a dandelion (a transit he compared to “traveling from London to Paris by way of Strasbourg steeple”). This ant has nothing on the indirection in Bach’s cascade. The initial modulation from A major to the remote key of G-sharp major hits the Kyrie’s prevailingly calm tonality like an impossibly powerful gust; from a position just off the piece’s home port of F-sharp minor, we find ourselves instantaneously blown far out to sea. But as in the previous two cascades, a chain of dominant-to-tonic progressions ensues, serving now as a series of tonal tacks that bear us indirectly but steadily home:

The passage we’ve been considering is, of course, my abovementioned nominee for Bach’s greatest moment. What makes it stand out for me is the sheer number of interrelated ways in which it so brilliantly works. There’s the cascade’s assumption of its optimal “waterfalls” form, the unparalleled height from which the cascade descends, the enormous (and syncopated) leap by which this height is attained, the inspired fusion of cascade motive and diminished third, the use of this fusion to power the stunning modulation from A major to G-sharp major, the structural stunningness of this modulation (given the proximity of A major to the Kyrie’s home key of F-sharp minor), and the use of the cadence-chain borrowed from the previous two cascades to tack tonally home in this one (a “navigational” adaptation as creatively inspired, in its way, as the fusion of diminished third and cascade motive). To consider so happy a confluence of so many elements is to wonder how it could ever have been brought about. Not that, as a statistical matter, the world won’t occasionally toss up instances of order so multifarious as to seem miraculous. But the tone-teeming world called Bach produced such seeming miracles on a considerably more regular basis.