_Columns._--Amongst the columns are beautiful examples of the dark green Thessalian marble, now called _verde antico_. Of this are formed all the round columns in the nave and ground-floor aisles, with the exception of the eight in the four exedras, which are of dark Theban porphyry. It could not have been always possible to find a sufficient number of columns of the same height and diameter, and the transport of them must have been frequently accompanied by injuries of one kind or another. There are differences between similarly situated columns, and in many cases mended fractures appear on the surface of the marble. In no cases are antique capitals placed on these columns. All the capitals and bases are of Proconnesian marble, and were wrought by Byzantine chisels. The greater part of the capitals are similar in design, though their size varies in proportion to the height of the columns which support them. Salzenberg, in Figs, 1 and 3 of his Plate xv., shows one of the capitals of the great order. The leaf-work on them--partly acanthus and partly palm--is very deeply undercut, and lies almost clear of the ground underneath. In the middle of front and back are monograms. Under the capitals are bronze rings eleven and half inches high; each is composed of three members, with a wrought lock on the side towards the nave, on which is repeated the monogram of each capital. At the foot of the columns above the bases are similar rings nine inches high. These rings occur on all the old columns, with the exception of the two dual columns of the west gallery. They seem to be let into the shaft, and, according to the description of the Silentiary, they were gilded. In addition to these rings, there are on other columns--as, for instance, the porphyry columns of the exedras--simple rings, rectangular in section, in positions where cracks and injuries appear; there being three or four such rings on a column at different heights. It is possible that some of these are of Turkish origin. The bases as a rule have much the same form as the Attic base; the porphyry columns of the exedras have pedestals[261] below them, because the shafts were not long enough. Each of the great verde antique shafts has a height of twenty-five and a half feet, and the bronze base-ring has an inside diameter of three feet seven inches. The capital is three feet ten inches high, and the upper part five feet eight inches wide, the whole height, including base and capital, being thirty-three and a half feet.