1911 Encyclopædia Britannica/Conglomerate

CONGLOMERATE (from the Lat. conglomerare, to form into a ball, glomus, glomeris; so also the general term “conglomeration” for a miscellaneous collection of things, gathered together in a mass), in petrology, the term used for a coarsely fragmental rock consisting of rounded pebbles set in a finer grained matrix. The pebbles must be rounded, otherwise the rocks are breccias, and these have a distinctly different geological significance. They have attained their present shapes by weathering and by attrition during transport by streams and the waves and currents of the sea. The pebbles consist mainly of hard rocks, such as granite, gneiss, sandstone, greywacke, or sometimes limestone. Quartzites, cherts and flints, and vein-quartz are among the hardest and most durable of all rocks, and hence are specially abundant in conglomerates. Fragments of vein-quartz form a large part of the “banket-rock” of the auriferous Transvaal reefs, one of the most important conglomerates economically. In this case the matrix consists mainly of quartz and chlorite, and gold occurs both in the matrix and in the pebbles. Igneous rocks on account of their toughness are also very abundant in many conglomerates; those at the base of the Old Red Sandstone of Scotland, which are thousands of feet in thickness, consist largely of andesite, porphyrite, granite, diorite and porphyry, along with vein-quartz, quartzite and various kinds of gneiss. Soft and friable rocks, on the other hand, such as shale, mica-schist and coal, are rarely found in any quantity as pebbles in conglomerate-beds. They are ground to pieces by friction against harder masses and help to form the matrix. The size of the pebbles varies greatly; occasionally they are 10 or 20 ft. in diameter, more frequently they are a foot or less. The cementing matrix in which the pebbles are embedded usually bears some resemblance in composition to the nature of the pebbles, but contains a larger proportion of the softer ingredients, such as clay, mica, weathered felspar, calcite and dolomite. Often it resembles a felspathic or calcareous sandstone; if limestone fragments are common it may be highly calcareous, or may be in large measure dolomitic. Often the matrix is stained red by compounds of iron. The “brockram” of the north of England is a well-known Permian limestone-conglomerate. The Dolomitic Conglomerate is a similar rock, but of Triassic age. Both of these are often extensively dolomitized and pass into breccias, where their fragments are angular and unworn. The pebble beds of the Bunter (Triassic period) are also familiar to geologists. They cover extensive areas in the midlands of England, and are well seen at Budleigh Salterton on the south coast. The pebbles are mostly quartzite with granite, chert, sandstone and igneous rocks.

Conglomerates are rarely well bedded, showing at most a rude stratification, but they may contain intercalations of finer materials such as sandstone and shale, which indicate the bedding clearly. In these fossils may be found, but they do not often occur in the conglomerates themselves, as the conditions are generally unsuitable for the preservation of organic remains. The pebbles, however, may be highly fossiliferous, and sometimes important evidence is provided by this means as to the age of the conglomerate. On account of the imperfect stratification it is often very hard to estimate the thickness of conglomerates, and this difficulty is increased by the fact that many of them must have been laid down as sloping banks of pebbles and not as flat layers of deposit. Conglomerates are merely consolidated gravels, and have originated mostly on seashores or in shallow waters near land. They are typical shore formations, and are especially frequent where one series of stratified rocks rest upon an older group unconformable. Other conglomerates occur along with fine-grained red sandstones, salt beds and such rocks as accompany desert deposits. We may compare them with the accumulations of pebbles which cover extensive areas of existing deserts. A quite distinct group of conglomerates characterizes regions where the rocks are much broken and sheared; these may very closely resemble true conglomerates, but have really been produced by the mashing together of rock masses along zones of fracture and movement. They are known as “crush-conglomerates” or “auto-clastic rocks.” Conglomerates may undergo metamorphism, and are then converted into “conglomerate-gneiss” or “conglomerate schist.” Their pebbles are flattened and dragged out of shape by interstitial movement, while the matrix becomes highly crystalline. One of the best-known examples of this is the Obermittweida gneiss (Saxony).  (J. S. F.)