The mosaic pavement in Westminster Abbey Presbytery is as fine an example of Roman Cosmati mosaic as one can see north of the Alps.  An inscription, almost obliterated, is interpreted by Mr. Lethaby as signifying, that in the year 1268 “Henry III. being King, and Odericus the cementarius, Richard de Ware, Abbot, brought the porphyry and divers jaspers and marbles of Thaso from Rome.”  In another place a sort of enigma, drawn from an arbitrary combination of animal forms and numbers, marks a chart for determining the end of the world!  There is also a beautiful mosaic tomb at Westminster, inlaid with an interlacing pattern in a ground of marble, like the work so usual in Rome, and in Palermo, and other Southern centres of the art.

While the material used in mosaic wall decoration is sometimes a natural product, like marble, porphyry, coral, or alabaster, the picture is composed for the most part of artificially prepared smalts—­opaque glass of various colours, made in sheets and then cut up into cubes.  An infinite variety in gradations of colour and texture is thus made possible.

The gold grounds which one sees in nearly all mosaics are constructed in an interesting way.  Each cube is composed of plain rather coarse glass, of a greenish tinge, upon which is laid gold leaf.  Over this leaf is another film of glass, extremely thin, so that the actual metal is isolated between two glasses, and is thus impervious to such qualities in the air as would tarnish it or cause it to deteriorate.  To prevent an uninteresting evenness of surface on which the sun’s rays would glint in a trying manner, it was usual to lay the gold cubes in a slightly irregular manner, so that each facet, as it were, should reflect at a different angle, and the texture, especially in the gold grounds, never became monotonous.  One does not realize the importance of this custom until one sees a cheap modern mosaic laid absolutely flat, and then it is evident how necessary this broken surface is to good effect.  Any one who has tried to analyze the reason for the superiority of old French stained glass over any other, will be surprised, if he goes close to the wall, under one of the marvellous windows of Chartres, for instance, and looks up, to see that the whole fabric is warped and bent at a thousand angles,—­it is not only the quality of the ancient glass, nor its colour, that gives this unattainable expression to these windows, but the accidental warping and wear of centuries have laid each bit of glass at a different angle, so that the refraction of the light is quite different from any possible reflection on the smooth surface of a modern window.