Blairsden, the 62,000 square foot 38-room Peapack, New Jersey mansion built by C. Ledyard Blair, heir to the Union Pacific fortune and a former resident of 740 Park, is on the market. Built in about 1900 at a cost of $2 million, it’s also a relative bargain at $4.9 million, even if it now sits on 34 acres compared to its original 423 acres.