Read "To A Mariposa Lily" at The Eldritch Dark:
As per the photo shown above, I'm guessing that in this poem Clark Ashton Smith (CAS) was referring to Calochortus clavatus, or the "clubhair mariposa lily" found widely in his native California, and particularly in the Sierra Nevada foothills. That particular species has yellow petals, which would seem to match the flower that CAS describes.
While this is a simple nature poem, it's quite effective. The opening phrase "Thou art the chalice of the sun" sets things up beautifully with the image of the flower as a goblet, waiting to be filled with the sun's radiance, wonderfully imagined as "the fountains of his light".