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
