CONGLOMERATE, in petrology, the term used for a coarsely fragmental rock consisting of rounded pebbles set in a finer grained matrix (from the Lat. conglomerare, to form into a ball, glomus, glotueris; so also the general term "conglomeration" for a miscellaneous collection of things gathered together in a mass). If the pebbles are angular, the rock is termed a breccia (q.v.). Conglomerate is thus merely consolidated gravel or shingle. The coarser constituents have attained their present rounded shapes by weathering and by attrition during transport by streams and the waves and currents of the sea. Their size varies considerably; occasionally they are io or loft. in diameter, but are usually a foot or less. Quartzites, cherts and flints, and vein-quartz being among the hardest and most durable of all rocks, are specially abundant in conglomerates; but granite, gneiss, sandstone and limestone are also of frequent occurrence. Although conglomerates occur in geological formations of all ages, they reach a considerable thickness only in the Palaeozoic rocks in Britain. In the old red sandstone of Scotland, for ex ample, they are thousands of feet in thickness and consist largely of boulders of andesite, granite, quartzite, gneiss, etc.
The matrix in which the pebbles are embedded is usually formed of sandy material cemented by a siliceous, ferruginous, calcareous, dolomitic or clayey bond. The "Brockram" of the North of England is a well-known Permian limestone-conglomerate. The Dolomitic conglomerate of the Bristol district is a similar rock of Triassic age. The pebble-beds of the Bunter (Trias) are a valuable source of underground water-supply. For Crush-con glomerate see BRECCIA. (P. G. H. B.)