Line in Milton's "Penseroso."

In those somewhat hacknied lines, "And may my due feet never fail," &c., I am somewhat puzzled to understand the expression, "With antique pillars massy *proof*." Now what is "proof,"—a substantive or adjective? If the latter, no edition is rightly stopped; for, of course, there should be a comma after "massy;" and then I somewhat doubt the propriety of "proof" for "proved," unless joined with another word, as "star-proof," "rain-proof." If "proof" is a substantive, "massy proof" is in apposition to "antique pillars," and is very meaningless. Can any of your readers suggest an explanation?