Complex sentence: Mellon was Secretary of the Treasury and the wealthiest man in America and had invested heavily in nylon, DuPont's new synthetic fiber, and considered its success to depend on its replacement of the traditional resource, hemp.
Split into simpler sentences: Mellon was Secretary of the Treasury, as well as the wealthiest man in America, and had invested heavily in nylon, DuPont's new synthetic fiber. He considered nylon's success to depend on its replacement of the traditional resource, hemp.