By the late 1890s, Mellon had amassed a substantial fortune, but his wealth paled in comparison to that of better-known business leaders such as John D. Rockefeller. In the late 1890s, the Union Trust Company emerged for the first time as one of Mellon's most important and profitable holdings after Mellon broadened the company's activities to include commercial banking. With an infusion of capital from Union Trust, Mellon underwrote the capitalization of National Glass and several other companies. He also became a co-owner of the McClintic-Marshall Construction Company and established Crucible Steel Company, the Pittsburgh Coal Company, and the Monongahela River Coal Company; the two coal companies collectively contributed 11 percent of the coal production of the United States. With Frick, Richard Mellon, and William Donner, Mellon co-founded Union Steel Company, which specialized in the production of nails and barbed wire. Though Frick had fallen out with steel magnate and long-time business partner Andrew Carnegie, Mellon received Carnegie's consent to venture into the steel industry. Responding to the growing emphasis on naval power in the aftermath of the Spanish–American War, Mellon and Frick also became major shareholders in the New York Shipbuilding Corporation.