Read "The Years Restored" at The Eldritch Dark:
This poem presents a rather morbid vision, as the speaker laments the inability to recover all that has been lost in years past. The heart of this verse seems to be these lines from the opening octet:
Of the Past's great sum,
Our hands reach but the symbols recondite
In those brief lines, we get to the heart of the matter - we can never really know the glories of past ages; at best we can uncover some dim idea of what went before. Certainly a universal truth, and expressed so eloquently, as only Clark Ashton Smith could.