xtured
one. Expansion and contraction of a stone under ordinary temperature
changes, and also under fire and freezing, must necessarily be known for
many kinds of construction.

Minerals resist weathering to different degrees, therefore the mineral
composition of a rock is another considerable factor in determining its
durability. Where pyrite is present in abundance it easily weathers out,
leaving iron-stained pits and releasing sulphuric acid which decomposes
the rock. Abundance of mica, especially where segregated along the
stratification planes, permits easy splitting of the rock under
weathering. Likewise the mica often weathers more quickly than the
surrounding minerals, giving a pitted appearance; in marbles and
limestones its irregular occurrence may spoil the appearance. Flint or
chert in abundance is deleterious to limestones and marbles, because,
being more resistant, it stands out in relief on the weathered surface,
interferes with smooth cutting and polishing, and often causes the rock
to split along the lines of the flint concretions. Abundance of
tremolite may also be disadvantageous to limestones and marbles, because
it weathers to a greenish-yellow clay and leaves a pitted surface.

The crushing strength of a rock has an obvious relation to its
structural uses. The rock must be strong enough for the specified load.
Most hard rocks ordinarily considered for building purposes are strong
enough for the loads to which subjected, and this factor is perhaps
ordinarily less important than the structural and mineral features
already mentioned.

It is often necessary to know the modulus of elasticity and other
mechanical constants of a rock, as in cases where it is to be combined
with metal or other masonry or to be subjected to exceptional shock.

The beauty and coloring of a rock are its esthetic rather than its
utilitarian features. They are particularly important in the
construction of buildings and monuments for public or ornamental
purposes.

CRUSHED STONE

The largest use of rock or stone is in the crushed form for road
building, railway embankments, and concrete, and the prospect is for
largely increased demands for such uses in the future. For the purpose
of road building, it is necessary to consider a stone's resistance to
abrasion, hardness, toughness, cementing value, absorption, and specific
gravity. Limestone cements well, but in other qualities it is not
desirable for heavy traffic. Shal