Martin Marietta Materials Inc on Monday made an unsolicited offer to buy rival Vulcan Materials Co for $4.6 billion in stock.

In a letter to Vulcan Chief Executive Donald James, Martin Marietta said it would pay 0.5 share of its stock for each Vulcan share. That means its bid is currently worth around $35.87 a share, a 12 percent premium to Vulcan's current value of $32 a share.

(Reporting by Michael Erman, editing by Gerald E. McCormick)