Vitrified brick roads consist of a foundation course of Portland cement concrete, broken stone or slag macadam, or of brick laid flat, the first named being by far the most generally used, and a wearing course of vitrified brick. =Vitrified Brick.=--Vitrified brick are made from clay of such a character that when heated to the required temperature they will fuse into a glassy texture. Brick roads are constructed on roads carrying the severest of traffic and the brick must therefore be tough and of high resistance to wear. Not all of the clays from which brick may be manufactured will produce a product suitable for road construction, and paving brick, even though truly vitrified, are of different degrees of durability, depending upon the nature of the clay and the care exercised in the manufacture. Paving brick are manufactured by the stiff mud process, which means that the clay is molded into form in a relatively dry condition. To accomplish this, considerable pressure is exerted in forcing the column of clay through the dies, which form the prism from which the brick are cut. If the clay is unsuitable in character or is not properly ground and mixed, the brick will possess planes of weakness between the various layers of clay which have been pressed together, and these planes, called laminations, are a source of weakness if too marked. It is usual to specify that the brick used for road surfaces shall be free from marked laminations. If the brick is not properly burned it will be only partly vitrified and therefore not of maximum durability. It is customary to specify that the brick shall show a glassy fracture indicating complete vitrification. Various defects of a minor nature occasionally develop in the brick during the successive steps in the manufacturing process. Check cracks resulting from the burning or from too rapid cooling are often encountered, but unless these are deep, that is 3/16 inch or more, they do not impair the wearing quality of the brick, nor indicate structural weakness. Kiln marks are formed on some of the brick due to the weight of the brick above in the kiln. These depressions are not objectionable unless the brick are so distorted that they will not lie evenly in the pavement. Spacing lugs or raised letters are formed on one face of the brick to insure sufficient space between the brick for the filler. These lugs or letters are not less than 1/8 inch nor more than 1/4 inch high and of such design that they will not obstruct the free flow of filler into the joints between the brick. Several varieties of paving brick are to be had, the difference being principally in the design or size. =Repressed Brick.=--In this type of brick the spacing lugs are formed by pressing the green brick, after it has been cut to size, into a mold on one face of which are recessed letters or other devices into which the clay is pressed, thus forming the spacing lugs.