There are many kinds of rock which appear hard and compact when first excavated, but upon exposure to the weather undergo a complete change, 227 developing into soft masses containing too much clay to allow the water to pass through readily. Where rock is scarce and gravel plentiful, the lower layer may be made of the heavier or coarser gravel, leaving the finer gravel for the upper layer, or boxing; but there is no doubt that the broken stone pitching makes the most efficient bottom layer. No gravel ballast should be used which is not free from clay or earthy sand. Wherever there are particles of earthy matter, sufficient to furnish nourishment for vegetable growth, weeds will quickly spring up, and once established are most difficult, if not impossible, to eradicate. The presence of weeds checks drainage, and gives an untidy appearance to the line, besides constantly occupying a large portion of the platelayers’ time in their removal. Clean cinders, free from dust or earth, are much used for upper ballast and boxing, and being lighter than gravel, are specially applicable for soft boggy ground. Burnt clay, broken into small pieces, has been largely adopted in districts where both rock and gravel were difficult to obtain. Chalk, furnace-slag broken small, crushed brick and sand, are frequently used as ballast. Sand is objectionable where there is high-speed traffic, as the finer particles rise in the form of dust and deposit themselves on the vehicles and machinery of the train. The water-tables, or side drains in the cuttings, should be cut below the formation level, and to a depth or width sufficient to take away all rain-water, or water arising from springs. Where the material of the cutting is of a loose friable nature, it may be necessary to protect the sides of the water-tables with low dry stone walls, as in Fig. 338; or glazed earthenware pipes may be laid, as in Fig. 339, with open joints, or with grate openings at regular intervals. In some cases substantial side-walls and invert are requisite to carry away the flow of water. Timber sleepers intended for the flange railroad should have the rail-seats grooved by machinery to ensure perfect accuracy in the position of the grooves, and in the angle or inclination of the rail-seats. Fig. 340 is a side view of part of a sleeper grooved to receive a flange rail. The presence of the grooves materially facilitate the laying of the rails to gauge, but must not be allowed to interfere with the constant use of the platelayer’s gauge. In a similar manner the timber sleepers for the chair road frequently have 228 the spike-holes bored to template by machinery, as indicated on Fig. 341. Steel or iron sleepers are delivered with the recesses for rails, and holes for bolts or fastenings formed complete by machinery.