A similarly variegated effect in red and white is produced by building the arches of windows and doors with alternating voussoirs in brick and marble.
The arches, round or more often pointed in form, were decorated with moulded terra-cotta enrichments, and often with alternating voussoirs of marble.
The arch ring can be treated as a blockwork structure composed of rigid voussoirs.
The voussoirs of the centre arch (all of granite) are 4 ft.
The difficulty of casting heavy arch ribs led to the construction of cast iron arches of cast voussoirs, somewhat like the voussoirs of masonry chambers and air locks, a feat unprecedented in the annals of engineering.