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It has been too many years since I read On The Road and do not remember these lines specifically. His use of poetic imagery here reminds me of a line by Archibald MacLeish: "A poem doesn't mean but be." Let us look at it from Kerouac's view. Jazz as an art form began just before the great depression and began to bud and blossom during the depression. The end of the roaring, happy twenties introducing the depressed thirties. This would have been Jazz' dawning and would span Jack's early years. Thus the holy (beatific) but tired faces. - Holy flowers floating in the air...
"Dust rose to the stars...sad music..." He wrote this book at the dawning of the atomic age. I can easily see this line as a metaphor of this sad event: The bombing of Hiroshima and Nagasaki.
After writing this I now have a strong urge to reread that book. Thank you for making me think some interesting thoughts tonight.
This is, of course, merely my conjecture as to the meaning of these lines. I am certainly not privey to Kerouac's intent. One gets from any work of art in proportion to what one brings to it. I was about 19 or 20 when I read this book. I would love to read it again because I have so much more to bring to it now. I am 69 years old.