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ECONOMIC GEOLOGY 
which absorbs carbon dioxide from the air and is deposited 
as carbonate of lime in the interstices of the stone and thus 
strengthens it. 
The process failed at Westminster because the decay had 
gone too far, and the new crust flaked away. The restora- 
tion of building stone which has undergone thorough decay 
has proved so far economically impracticable. 
II. Roap METALS 
The suitability of stones for road construction depends 
on qualities different from those required in building and 
varies with the climate and traffic. A good road metal must 
be sufficiently tough to withstand the traffic; the powder 
formed by its wear and tear should act as a cement and form 
a smooth impermeable surface; and the stone and its 
powder should hold by surface-tension the water mixed with 
it, and the tar or bitumen added as binding material. The 
selection of road metal depends primarily upon trafic. A 
comparatively soft stone will carry light vehicles if its 
powder binds well; hence limestone, volcanic tuff and 
laterites, though weak and friable, form excellent country 
roads. Massive granite setts may be necessary to carry 
heavy lorries near docks and factories. For such setts 
hardness alone is inadequate, for a quartzite would become 
slippery and make a good slide but a bad pavement; rocks 
are used which have constituents of different hardness, such 
as granite, as the felspars wear more readily than the quartz, 
and the surface keeps rough and gives a good grip. As a 
rule, however, the constituents of a stone should be nearly 
2qual in hardness. 
Paved roads were built by the Romans, and are still used 
in China; they are suitable for pack animals, but not for 
wheeled traffic, as adjacent slabs inevitably settle at different 
levels and the fall of the wheel on to a lower slab drives it 
lower and may break it. Most modern roads have a surface 
of macadam, so called from the adopted name of its inventor : 
it consists of pieces of tough stone of uniform size, about 
I or 2 inches in diameter, mixed in some binding material ; 
this layer rests upon a foundation which admits of some 
vibration, so that the surface is elastic and yields slightly to 
a heavy shock.