The poem
Now as at all times I can see in the mind's eye, In their stiff, painted clothes, the pale unsatisfied ones Appear and disappear in the blue depth of the sky With all their ancient faces like rain-beaten stones, And all their helms of Silver hovering side by side, And all their eyes still fixed, hoping to find once more, Being by Calvary's turbulence unsatisfied, The uncontrollable mystery on the bestial floor.
I think William Butler Yatesis refering to clouds in this poem