The filling between the girders and floor beams consists of segmental arches of brick, segmental or flat arches of porous (sawdust) terra-cotta, or hard-burned hollow terra- - cotta voussoirs, or various patented forms of con crete floors containing ties or supports of steel or iron.
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.
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.