Source of Cage quote about Ives...

Pwyll ap Sion (senior lecturer at Bangor University) is looking for the source of this quote by Cage:

'What interests me are not the Americana aspects, the tunes and all that, but what I call the mud, the complexity of many things going on at once, in which I am not able to know where I am, or what's happening. Invariably in this mystery, something begins to happen to my mind, to change it, because of what I'm hearing. But in my experience, that 
change of my mind is interrupted by the emergency from the mud of some well-known tune, generally some Protestant church tune, and I find myself in a place familiar to others, but carefully avoided by me, in the land so to speak of melodies and accompaniments or, I suppose Ives would prefer it if he's listening, if I would say melodies and 
precedents."

Michael Nyman used this quote in a paper at the First American Music Conference, Keele University, in 1975, and mentions the this viewpoint of Cage is more developed from Cage's "Two Statements on Ives" in A Year from Monday, so we figure it must come in between 1967 and 1975. A few times Cage mentions Ives and mud in the same breath, but not "Protestant church tune" that I can find. I am guessing his mention of church tunes comes from his work on Apartment House/Renga, but that is just my guess.


Thanks for any help,

-Josh Ronsen
http://ronsen.org