[Footnote 1: There are at least four definite cleavages at Coniston, besides joints. One of these cleavages furnishes the Coniston slate of commerce; another forms the ranges of Wetherlam and Yewdale crag; a third cuts these ranges to pieces, striking from north-west to south-east; and a fourth into other pieces, from north-east to south-west.] B. And then, even if we know what the quarry bedding is, how are we to keep it always in our building? You may lay the stones of a wall carefully level, but how will you lay those of an arch? You think these, perhaps, trivial, or merely curious questions. So far from it, the fact that while the bedding in Normandy is level, that at Carrara is steep, and that the forces which raised the beds of Carrara crystallized them also, so that the cleavage which is all-important in the stones of my garden wall is of none in the duomo of Pisa,--simply determined the possibility of the existence of Pisan sculpture at all, and regulated the whole life and genius of Nicholas the Pisan and of Christian art. And, again, the fact that you can put stones in true bedding in a wall, but cannot in an arch, determines the structural transition from classical to Gothic architecture. 154. The _structural_ transition, observe; only a part, and that not altogether a coincident part, of the _moral_ transition. Read carefully, if you have time, the articles 'Pierre' and 'Meneau' in M. Violet le Duc's Dictionary of Architecture, and you will know everything that is of importance in the changes dependent on the mere qualities of _matter_. I must, however, try to set in your view also the relative acting qualities of _mind_. You will find that M. Violet le Duc traces all the forms of Gothic tracery to the geometrical and practically serviceable development of the stone 'chassis,' chasing, or frame, for the glass. For instance, he attributes the use of the cusp or 'redent' in its more complex forms, to the necessity, or convenience, of diminishing the space of glass which the tracery grasps; and he attributes the reductions of the mouldings in the tracery bar under portions of one section, to the greater facility thus obtained by the architect in directing his workmen. The plan of a window once given, and the moulding-section,--all is said, thinks M. Violet le Duc. Very convenient indeed, for modern architects who have commission on the cost. But certainly not necessary, and perhaps even inconvenient, to Niccola Pisano, who is himself his workman, and cuts his own traceries, with his apron loaded with dust.