BUILDING STONES AND ROAD METALS 183 
The stones used in macadam must be tough so as to resist 
compression and sudden blows. The best road metals are 
igneous rocks in which the constituents are intergrown, and 
especially those containing a fibrous or prismatic mineral 
such as hornblende. The pyroxenes are less useful as their 
grains are torn apart, and they yield more readily along the 
cleavage. Large felspars also break along the cleavages. 
Among the best of the acid rocks are the granophyres in 
which the base consists of a fine-grained micropegmatitic 
intergrowth of quartz and felspar. The basic rocks have 
the advantage that being heavy, pieces are less easily dis- 
placed, though a given weight of stone covers a smaller area ; 
those with an ophitic structure, such as dolerite, are the 
best. 
The sedimentary rocks are usually less satisfactory as 
their rounded grains are easily torn out of the cement, and 
powdered quartz has a low cementing value. Limestone, 
though soft, has the advantage that its powder acts as a 
natural cement. Coarse gritty sandstones, such as the gray- 
wacke of the Southern Uplands of Scotland and the harder 
seams in the Old Red Sandstone of the North of Scotland 
serve as fair road metal; they are known as whin—a term 
given to any rock that was difficult to quarry or resisted 
decay into soil. The term is now often used as if applicable 
only to igneous rocks. 
An important factor in road metal is its adherence to tar 
and bitumen; hence granite is unsuitable for macadam, as 
if overheated the cleavages open on cooling and the rock 
becomes friable ; if inadequately heated the tar peels off and 
does not bind the material properly under the vibration of 
traffic. 
Road metal is tested by two methods—abrasion against 
a revolving iron plate, and the attrition test, by loss of 
weight when road metal is rotated on a cylinder. Love- 
grove’s test (Attrition Tests of Road-making Stones, with 
Petrological Descriptions, by J. S. Flett and J. A. Howe, 1905), 
often adopted in the London district, determines the per- 
centage of material that will pass through a sieve with spaces 
one-sixteenth of an inch in diameter after 16 pieces of the 
stone which together weigh 4 Ib. have undergone 5 hours 
rotation. at the rate of 20 revolutions per minute, in an