FooK II. ROADS OF VARIOUS ENGINEERS. 587 



3636. Stone is universally allowed to be the best kind of material for roads ; and granite, 

 trap, or flint, the best species of stone ; next in order are some sorts of limestone, and hard 

 sandstone. Soft claystone is the worst. Limestone is the principal material in Wiltshire, 

 Somersetshire, Gloucestershire, and Ireland ; granite and trap in the north of England 

 and Scotland ; slatestone in North Wales ; sandstone pebbles in Shropshire and Stafford- 

 shire ; flint in Essex, Sussex, and part of Kent ; and gravel in Middlesex and Surrey. 

 The stones used for the metals of any road," Paterson observes, " should always be 

 the hardest and most durable that the place or neighbourhood can afford. But this dura- 

 bility will be found in a great measure to depend on the dryness of the road. Freestone, 

 of a moderate hardness, such as mineralogists would term No. 6., 'that would with 

 difl[iculty yield to the knife,' will make a very good road on a dry sloping bank, exposed 

 to the sun and air, or even on a level surface that has a dry gravelly bottom. Nay, even 

 seven or eight inches deep of such metals, on such situations, will make a better road 

 than twelve inches of the best metals where the bottom is constantly damp, and will 

 actually surpass them in point of durability. This, however, is not meant to give a pre- 

 ference to those metals, but merely to show the great difference there is betwixt a wet 

 and a dry bottom ; and that such metals will answer very well in the situations above 

 described. Still it must be held as a general rule, to take the best and hardest metals the 

 neighbourhood can afford, as formerly mentioned." 



36S7. But the hardest metals will not always be found the most durable ; and here it may 

 be remarked, as another general rule, with some exceptions, that the harder they are to 

 break, the greater their durability. Some stones, for instance, as hard as No. 9. of mine- 

 ralogists, " such as would give a few feeble sparks with steel," are so free that they will 

 fly under the stroke of a hammer like so many pieces of glass. These, although very hard, 

 being of a quality so free and brittle, will grind down by the wheels rather easily, and in 

 time of rains will be formed into mud ; while, on the other hand, there are stones not 

 harder than No. 7. that are so tough, that there is great difficulty in breaking them. 

 Yet these latter, although two degrees softer, will absolutely last longer than the former, 

 on any road whatever. 



3638. Flints reduced to a small size, and mixed with chalk, make an excellent road 

 in dry weather ; but chalk being very absorbent of water, they become slippery and soft 

 in moist weather, and are much affected by frost. 



3639. Whinstone, M'Adam and all road engineers agree in considering the most 

 durable of all materials ; and, wherever it is well and judiciously applied, the roads are 

 comparatively good and cheap. Fry, however, has uniformly observed, in various parts, 

 of England, that where limestone is used, the roads are the best ; and this superiority is 

 not in his opinion owing merely to the hardness of this substance, but also to its adhesive 

 or cementing projjerty : how otherwise, he says, are we to account for the firmness and 

 solidity of the roads around Bristol, that are made of white limestone. Fall mentions 

 dewstone, which abounds in Nottinghamshire and other counties of the North, as equally 

 durable with whinstone. {Every Man his own Road-maker, p. 8.) 



3640. Gravel is of two kinds ; that obtained from pits, and that from the beds of rivers. 

 Gravel is generally silicious and hard ; otherwise, indeed, it would have been worn down 

 to sand, in undergoing the operation with has rendered it gravel. This material is chiefly 

 used on the roads round London : it is often found, Paterson observes, " to answer 

 very well in point of durability. But such kind of gravel, being composed chiefly of 

 hard sand, and smooth, little, round stones, does not so easily bind together, and seldom 

 makes a very firm road. On the other hand, stones that are broken have so many sides, 

 that they readily lock into one another ; whereas the small round gravel keeps rolling and 

 shifting about by every motion of the wheels. All road metals, therefore, should be 

 of stones as large as to require breaking before they are used. The roads on which 

 gravel will be found to answer best, are those which are neither too wet nor too dry. I 

 have seen a road made with such materials, not only easily rutted in time of the winter 

 rains, but the same road, in the drought of summer, became as loose as ashes, and was 

 then, also, very easily rutted ; while betwixt these extremes it answered exceedingly well. 

 Upon the whole, it would be improper to use gravel for any turnpike or public road, where 

 stones can be got that require to come under the hammer." {Treatise, ^c. p. 31.) 



364 1 . The gravel of which roads are usually formed is mixed with a large portion of clay ; 

 and the component parts of gravel are round, and want the angular points of contact by 

 which broken stone unites and forms a solid body : the loose state of the roads near 

 London is a consequence of this quality in the material, and of the entire neglect or 

 ignorance of the method of amending it. 



3642. Gravel is the rvorst material for making roads subject to great traffic Telford, 

 on being asked his opinion of it by the road committee, replied, " I am of opinion that 

 the materials in the whole valley or plain round London being entirely silicious, or flints, 

 and easily ground to dust, are very improper. This must be evident to every person who 

 travels near London in any direction." In this opinion M'Adam concurs. 



