es of a book of this scope.[13]

BUILDING STONE

For building stones, the principal geologic features requiring attention
are structure, durability, beauty, and coloring.

The structures of a rock include jointing, sedimentary stratification,
and secondary cleavage. Nearly all rocks are jointed. The joints may be
open and conspicuous, or closed and almost imperceptible. The closed
joints or incipient joints cause planes of weakness, known variously as
rift, grain, etc., which largely determine the shapes of the blocks
which may be extracted from a quarry. Where properly distributed, they
may facilitate the quarrying of the stone. In other cases they may be
injurious, in that they limit the size of the blocks which can be
extracted and afford channels for weathering agents. Some rocks of
otherwise good qualities are so cut by joints that they are useless for
anything but crushed stone. The bedding planes or stratification of
sedimentary rocks exercise influences similar to joints, and like
joints may be useful or disadvantageous, depending on their spacing. The
secondary cleavage of some rocks, notably slates, enables them to be
split into flat slabs and thus makes them useful for certain purposes.

Proper methods of extraction and use of a rock may minimize the
disadvantageous effects of its structural features. The use of
channelling machines instead of explosives means less shattering of the
rock. By proper dressing of the surface the opening of small crevices
may be avoided. Stratified rocks set on bed, so that the bedding planes
are horizontal, last longer than if set on edge.

The durability of a rock may depend on its perviousness to water which
may enter along planes of bedding or incipient fracture planes, or along
the minute pore spaces between the mineral particles. The water may
cause disastrous chemical changes in the minerals and by its freezing
and thawing may cause splitting. For this reason, the less pervious
rocks have in general greater durability than the more pervious. Highly
pervious rocks used in a dry position or in a dry climate will last
longer than elsewhere.

Durability is determined also by the different coefficients of expansion
of the constituent minerals of the rock. Where the minerals are
heterogeneous in this regard, differential stresses are more likely to
be set up than where the minerals are homogeneous. Likewise a
coarse-textured rock is in general less durable than a fine-te