Read "September" at The Eldritch Dark:
I can't identify the poetic form that Clark Ashton Smith (CAS) is using here (at a superficial level it's reminiscent of a limerick). It's an aabbaa rhyme scheme where two "a" lines in each stanza have end rhymes on the same word. The third stanza is where this unusual rhyme scheme really shines:
Embers from a dreamland hearth,
Glow the leaves in croft and garth;
Vines within the willows drawn
Relume the gold of visions gone;
Darkly burn, in croft and garth,
Embers from a dreamland hearth.
The pattern of refrain used in the opening and closing couplets of each stanza lends a chant-like cadence to the reading of "September", which seems quite appropriate for a poem that revels in the diminishing sunlight of the autumn season.