some
distance. There are many ways in which breccias may originate. Some are
formed by ordinary processes of atmospheric erosion; frost, rain and
gravity break up exposed surfaces of rock and detach pieces of all
sizes; in this way screes are formed at the bases of cliffs, and barren
mountain-tops are covered with broken debris. If such accumulations
gather and are changed into hard rock by pressure and other indurating
agencies they make typical breccias. Conglomerates often pass into rocks
of this type, the difference being merely that the fragments are of
purely local origin, and are unworn because they have not been
transported. In caves breccias of limestone are produced by the collapse
of part of the roof, covering the floor with broken masses. Coral reefs
often contain extensive areas of limestone breccia, formed of detached
pieces of rock which have been dislodged from the surface and have been
carried down the steep external slopes of the reef. Volcanic breccias
are very common near active or extinct craters, as sudden outbursts of
steam bear fragments from the older rocks and scatter them over the
ground.

Another group of breccias is due to crushing; these are produced in
fissures, faults and veins, below the surface, and maybe described as
"crush-breccias" and "friction-breccias." Very important and well-known
examples of this class occur as veinstones, which may be metalliferous
or not. A fissure is formed, probably by slight crustal movements, and
is subsequently filled with material deposited from solution (quartz,
calcite, barytes, &c.). Very often displacement of the walls again takes
place, and the infilling or "veinstone" is torn apart and brecciated. It
may then be cemented together by a further introduction of mineral
matter, which may be the same as that first deposited or quite
different. In important veins this process is often repeated several
times: detached pieces of the country rock are mingled with the
shattered veinstone, and generally experience alteration by the
percolating mineral solutions. Other crush-breccias occurring on a much
larger scale are due to the folding of strata which have unequal
plasticities. If, for example, shales and sandstones are bent into a
series of arches, the sandstones being harder and more resistant will
tend to crack, while the shales, which are soft and flow under great
pressures, are injected into the crevices and separate the broken pieces
from one anothe