where ashes take the forms of houses and chimneys and rising smoke and, finally, with a transcendent effort, of men who move dimly and already crumbling through the powdery air.
=>
where ashes take the forms of men who move dimly through the powdery air。 Those men are already crumbling。
把“already crumbling” 理解成为省略的定语从句来修饰前面的“men” ， 也就是： and men who already crumbling.
Those men are already crumbling (two possible interpretations: one is that those men themselves are, like ashes, decaying; or those men are like fallen bits of ashes if you look at them from distance.)
Jus my two cents.
