The nave terminates in a large apse, semicircular within and showing three sides on the exterior. Only the lower part is original; the Turkish superstructure is lower and on a smaller scale than the Byzantine portion it has replaced. There are no side chapels. Under the bema the Russian explorers discovered a small cruciform crypt. The large quantity of mosaic cubes found in the church during the recent Russian excavations proves that the church was decorated with mosaics, while the remains of iron plugs in the western wall for holding marble slabs show that the building had the customary marble revetment. But what is curious is to find the mortar pressed over the face of the stones, and broad decorative joints formed by ruled incised lines and colour. Mr. W. S. George suggests that this was a temporary decoration executed pending some delay in the covering of the walls with marble. He also thinks that the importance given to the joint in late Byzantine work and in Turkish work may be a development from such early treatment of mortar. The floor of the church was paved with pieces of marble arranged in beautiful patterns, in which figures of animals and scenes from classic mythology were inlaid. Gerlach[77] noticed the beauty of the pavement, and Salzenberg[78] represents a portion of it in his work on S. Sophia. But the members of the Russian Institute of Constantinople have had the good fortune to bring the whole pavement to light. A noticeable feature is the number of doors to the church, as in S. Irene. Besides the five doors already mentioned, leading into the interior from the narthex, there is a door at the eastern end of each aisle, and close to each of these doors is found both in the southern and northern walls of the building an additional door surmounted by a window. The latter doors and their windows have been walled up. The exterior is in two stories, corresponding to the ground floor and the galleries. It has two ranges of eight large semicircular-headed windows in the northern and southern walls, some of them modified, others built up, since the building became a mosque. The five windows in the gable of the western wall are, like the wall itself, Turkish. Pilasters are placed at the angles and at the apse. On the south side of the church is a cistern, the roof of which rests on twenty-three columns crowned by beautiful Corinthian capitals. NOTE The full text of the description given of the church of S. John, mentioned by Ruy Gonzalez de Clavijo, reads as follows:--