§ 1. No subject has been more open ground of dispute among architects than the decoration of the wall veil, because no decoration appeared naturally to grow out of its construction; nor could any curvatures be given to its surface large enough to produce much impression on the eye. It has become, therefore, a kind of general field for experiments of various effects of surface ornament, or has been altogether abandoned to the mosaicist and fresco painter. But we may perhaps conclude, from what was advanced in the Fifth Chapter, that there is one kind of decoration which will, indeed, naturally follow on its construction. For it is perfectly natural that the different kinds of stone used in its successive courses should be of different colours; and there are many associations and analogies which metaphysically justify the introduction of horizontal bands of colour, or of light and shade. They are, in the first place, a kind of expression of the growth or age of the wall, like the rings in the wood of a tree; then they are a farther symbol of the alternation of light and darkness, which was above noted as the source of the charm of many inferior mouldings: again, they are valuable as an expression of horizontal space to the imagination, space of which the conception is opposed, and gives more effect by its opposition, to the enclosing power of the wall itself (this I spoke of as probably the great charm of these horizontal bars to the Arabian mind): and again they are valuable in their suggestion of the natural courses of rocks, and beds of the earth itself.