That’s the slightly-rusted bottom flange of an old I-beam, surrounded by more or less modern concrete that replaced the original brick vault floor. The end of the beam is a bit rusted – not severely, not necessarily enough to require repair – but enough to show that it’s wrought iron.
One of the questions that comes up again and again when dealing with nineteenth-century buildings is how do we distinguish steel from wrought iron from cast iron. Cast iron is the easy one: because the metal was literally sand-cast, it typically has very small-scale pebbling of its surface. You might have to remove a quarter-inch of paint to see it, but the surface is slightly rough with a directionless pattern.
Wrought iron and steel are tougher to tell apart because of their many similarities. Both metals were rolled into shapes – plates, angles, channels, zees, and I-beams – using the same technology and often in the same mills. Both have a microscopic crystalline structure consisting of iron and other elements. Both have, in their undamaged state, smooth surfaces. I can’t tell the difference between steel and wrought iron by sight if they’re undamaged. The visual clues are simply too subtle. It’s always possible to send a piece to a lab for examination, of course, or I can look for the kind of damage seen in the photo above.
The rusted surface is pitted, without delamination. Some of the pits are very small circular depressions in the surface, but most are longer, oriented longitudinally along the beam. That’s a wrought iron rusting pattern. It’s not conclusive, particularly if the building in question was built in the 1880s when the chemical differences between wrought iron and steel might not be very big, but it’s a good first-pass assumption.
In this case, I’m cheating: I know that the beam is wrought iron. That’s why I’m using a picture of it to illustrate the point.