Two Questions About the Sixteenth Epode

Extract

I suppose we have all at some time been puzzled by Horace's substitution of boulders for the iron mass that the Phocaeans threw into the sea when they took their oath, and have wondered what poetical purpose the boulders could serve that iron could not. Would not iron in fact better cohere as an image with all the civil war that fills the poem's opening lines and with the agreeable absence of plough-shares and pruning-hooks from the Blessed Isles? Most of all, would it not cohere better with the poem's closing lines?