STONE BLOCK PAVEMENTS are most commonly of granite, some of the harder and tougher sand stones, and trap rock, the latter being the least satisfactory of the three. The lengths and depths are quite variable in different cities, but in the hest work the widths range from 3 to 4 or inches, with a tendency to 3% inches in Ameri can cities. The governing condition of width is the foothold for horses. Depths should be at least one and one-half times the narrow and me dium widths, to give a firm setting that will pre vent rocking on the base. In American cities common dimensions are as follows: Widths, 3y, to 4 inches; lengths, 9 to 12 inches; depths, 6 to 8 inches. In a number of European cities there is a tendency to square or nearly square blocks, ap proximating 6 inches on a side, with depths of 6 to S inches. Square blocks (or Belgian: see below) are also used in the United States, but not frequently of granite, as is the case abroad. The blocks should be as nearly rectangular and that as is possible on the tops, sides, and ends. The blocks are generally laid crosswise of the street, and diagonally at intersections. A sand cushion is used when the blocks are laid on con crete, as they should be almost invariably, since stone blocks are generally used where the traffic is heavy. The filler most commonly employed for the joints is first gravel, then hot paving cement. Medina sandstone is sometimes laid as close as possible, and filled with asphaltic cement, Portland cement, or Murphy grout. Belgian so named from their use in Belgium, have tops five or six inches square, bottoms somewhat less, but generally not more than one inch smaller each way, and depths of seven to eight inches.
These blocks have been extensively used near New York, and trap rock, being plentiful and cheap in that vicinity, has been employed for the purpose. The trap does not break to a true surface readily and is not easily worked, so it is difficult to get good surfaces with Belgian block of this material. It is very durable, hut wears smooth and slippery. Belgian blocks are almost if not quite always laid on a sand or earth base, since one of the objects in their selection is cheapness; their joints are filled with sand. Trap-rock blocks are also got out and laid in oblong form, much like granite and sandstone in general outline, but with far less regular sur faces. Limestone has been used for block pave ments, but to a small extent, as it is too soft to be durable.
All blocks should he laid diagonally at street intersections, as is done with brick. The se curing and dressing of stone blocks does not differ materially from the quarrying and working of other dimension stone. Cross-rya/Ls are laid at street intersections. They are composed of two parallel rows of bluestone, granite, or sandstone, according to the locality and the kind of pave ment, with a row- of blocks between. The cross stones are to 2 feet wide. 4 to 6 feet long, and 6 to S inches thick. In the best modern work they are laid with a keystone at the centre of the street, and with diagonal joints each way, at right angles to the direction of the traffic in turn ing corners.