Occasionally, but by no means frequently, small pieces of charcoal are mixed with the mortar. These have evidently been derived from the wood used in burning the lime. Excepting in the buildings of the stations, pounded tile, so characteristic of the Roman mortar in the south of England, is by no means a common constituent of the mortar of the Wall. Limestone is abundant in most parts of the district through which the Wall passes. The Romans probably burnt it in ‘sow kilns.’ The limestone and fuel being arranged in alternate layers, the whole was carefully covered with turf and ignited. This simple method is still much resorted to when the lime is wanted for farm purposes. _PLATE III_ [Illustration: Sections and Elevations of the Masonry of the Wall] [Sidenote: THE MODE OF BUILDING.] Supposing the stones to be now quarried and squared, the lime burnt and mixed with sand and gravel, the next point to be attended to is the method of using them. The foundation has been prepared by the removal of the natural soil to the width of about nine feet. In the hill district, a very scanty portion of earth covers the rocks; in the richer regions an excavation of from fifteen to eighteen inches has been made before the subsoil was reached. On the outer and inner margins of the ground thus bared, two rows of flags of from two to four inches in thickness, and from eighteen to twenty in breadth, were generally laid; no mortar was placed under them.[47] On these lay the first course of facing-stones, which were usually the largest stones used in the structure. In higher courses the facing-stones are uniformly of free-stone, on the ground course a ‘whin-stone’ is occasionally introduced. The flagstones of the foundation usually project from one to five inches beyond the first course of facing-stones, and these again usually stand out an inch or two beyond the second course, after which, the wall is taken straight up. In some parts of the line the flagstones do not appear in the foundation—the first course of facing-stones being laid directly upon the ground. In the neighbourhood of Sewingshields, where large tracts of the Wall have been recently removed, a careful observer informs me, that the entire foundation has for some distance been laid upon a bed of clay of three or four inches thick. [Sidenote: THE RUBBLE OF THE WALL.]