I was contacted recently by a reader, a priest who had concelebrated a Station Mass at San Marco di Campidoglio in Rome. He had notice a square halo on one of the figures in the 9th century mosaic in the apse and wanted to know the reason for this. The figure, he told me, was Pope Gregory IV. A square halo signifies that the person portrayed was alive at the time of its creation. Although this church was founded long before Pope Gregory himself lived, he was responsible for much rebuilding, the bulk of what we see today. This is why, I am guessing, he is also portrayed carrying a church. We see also a very clear example of what my teacher Aidan Hart always referred to as hierarchical perspective. The most important figure portrayed, Christ, is by far the largest. Also we see the Evangelist himself is putting his arm around the Pope in a touching detail that gives more of a sense of personal tenderness than one normally associates with iconographic mosaics of this type.
This is church is not always open to the public so it is interesting to have this insight into the interior.