Some road-makers use a pavement even on a substratum of rock. where it is uneven, but in many cases it is unnecessary ; although, if the surface be smooth, it should be picked to a de,gree of roughness similar to that of a pavement, in order that the road materials may not slide upon it. Where paving the whole width of a road might be too expensive, the pavement is sometimes limited to a width of 16 or 18 feet in the centre. in situations where coarse stone of suitable quality can be easily procured, it is found to be cheaper to make a road with 6 inches of broken stone and a pavement, than with 10 inches depth of broken atone without paving. Mr. Wingrove, surveyor of the Bath roads, mentions the use of freestone brash, chalk, &c., for ' forming the foundation pavement of metalled roads. Of late years burnt clay balliat has been substituted for rough stone pavements in the brick earth or blue clay districts around London, with considerable success.
The quality and right application of the road-metal, or broken stone, which forms the surface of the road, is of great importance. As a general rule, the hardest stone is to be preferred; but this rule admits of some qualification, some very hard stones being found to wear much more rapidly than others of a softer but tougher quality. According to Parnell, whose experience rendered him a good authority, the best descriptions of road materials " consist of basalt, granite, quartz, syenite, and porphyry rocks." The whinstones found in different parts of the United Kingdom, Guernsey granite, Mounteorrel and Hartshill stone of Leicestershire, and the pebbles of Shropshire, Staffordshire, and Wanvickshiro, are among tho beat of the stones now commonly in use. The schistus stones will make smooth roads, being of a slaty and argillaccous structure, but they are rapidly destroyed by wet, by the pressure of wheels, and they occasion great expense in scraping and constantly laying on new coatings. Limestone is defective in the same respect. It wears away rapidly when wet, and therefore, when the traffic is very great, it is an expensive material. Sandstone is much too weak for the surface of a road ; it will never make a hard one, but it is very well adapted to the purpose of a foundation pavement. Flints vary very much in quality as a road material. The hardest of them are nearly as good as the best limestone, hut the softer kinds are quickly crushed by the wheels of carriages, and make heavy and dirty roads. Gravel, when it consists of the pebbles of the hard sorts of stones, is a good material, particularly when the pebbles are so large as to reknit of their being broken ; but when it consists of limestone, or sandstone, it is often a very bad one ; for it wears so rapidly that the crust of p road made with it always consists of a largo portion of the earthy matter to which it is reduced. This prevents the gravel from be
coming consolidated, and renders a road made with it extremely defective with respect to that perfect hardness which it ought to have." Mr. Stevenson, in the article 'Road,' in the Edinburgh Encyclopaedia; states the distribution of road materials in the British islands to bo partial and irregular. " Throughout Scotland, and even as far south as the approaching sources of the rivers Tees and Ribble, good road metal is generally to be met with, containing the numerous varieties of granite, greenstone, basalt, porphyry, and limestone. South of this boundary, as far as the Trent and the Deo in Cheshire, the formation Is chiefly carboniferous sandstone, and the softer varieties of limestone. In the southern counties chalk and gravel soils chiefly occur, affording flint and gravel, both of which, under proper management, make ex cellent reads. In North and South Wales we have all the varieties of road-metal which are common to Scotland. In Ireland they have excellent road materials, as granite and limestone are pretty generally distributed." An interesting experiment has been trios on a part of the Holyhead road between London and Birmingham, as to the effect of iron amongst the road-metal in diminishing the wear of the road. The iron is cast In the form of cubes, about an inch square, and when the road was consolidated, holes large enough to receive them were picked in its surface. A single cube was then placed in each hole, so AS to be level with the road', and the small stone-chips were beat down about the iron with a mallet. One of these iron cubes is placed in every four inches of surface. They very soon become firmly imbedded, so as not to bo disturbed by the rolling of carriages or the feet of the horses ; and to assist their consolidation, it is recommended to water the road freely, if the cubes should be inserted in dry weather. The iron was applied in March, 1835, since which time the portion of road in which they are used has continued in excellent repair, and the wear is materially diminished. Mr. Macneill, the patentee of this method of road-making, considered it particularly applicable to streets, on account of its durability, and believed that the expense would' be trifling, as iron of the worst quality might be used. It may be observed that the draught on the piece of road on which the ex periment is made is very easy, and that horses do not show any tendency to slip upon it.