It's a staircase - but what a staircase. This is the Great Western Staircase from the New York State Capitol Building in Albany, built in 1897. It was obviously built to impress the visitor with the magnificence and power of the state.
The staircase is enormous: 119 feet in height and containing 444 steps. The predominate materials are Corsehill freestone, medina sandstone, limestone, and granite. It is illuminated throughout by light fixtures designed by Louis Hinton, and an enormous skylight on the top floor bathes the uppermost levels in natural light.To save on my allotment of space at the bloghost, I've just embedded this small image; if the architecture has any interest at all for you, you need to view the supersized photo at Shorpy, where you can explore the marvelous intricacies of the stonework.
Modern photos (after recent restoration) are at this link.