BRECCIA, in petrology, the name given to rocks consisting of angular fragments embedded in a matrix. They may be com posed of any kind of material, and the matrix, which usually corresponds to some extent to the fragments it encloses, may be siliceous, calcareous, argillaceous, etc. The distinctive character of the group is the sharp-edged and unworn shapes of the frag ments; in conglomerates the pebbles are rounded and water-worn. Breccias may originate in many' ways. 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 are changed into hard rock by pressure cementation they make typical breccias. Caves, coral reefs and volcanic regions are other frequent sources.
Another group of breccias is due to crushing; these are pro duced in fissures, faults and veins, below the surface, and may be described as "crush-breccias" and "friction-breccias." Very important and well-known examples of this class occur as vein stones, 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, etc.). 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. Other crush-breccias occurring on a much larger scale are due to the folding of strata which have unequal elastic ities. Great masses of limestone in the Alps, Scottish highlands, and all regions of intense folding are thus converted into breccias. Cherts frequently also show this structure; igneous rocks less commonly do so; but it is perhaps most common where there have been thin-bedded alternations of rocks of different character, such as limestone and dolerite, limestone and quartzite, shale or phyllite and sandstone. Fault-breccias closely resemble vein-breccias.
A third group of breccias is due to movement in a partly con solidated igneous rock, and may be called "fluxion-breccias." Lava streams, especially when they consist of rhyolite, dacite and some kinds of andesite, may rapidly solidify, and then become exceed ingly brittle. If any part of the mass is still liquid, it may break up the solid crust by pressure from within and the angular frag ments are enveloped by the fluid lava. When the whole comes to rest and cools, it forms a typical "volcanic fluxion-breccia." (J. S. F.)