DOLERITE is a word which has carried more than one meaning. It was originally applied by Haiiy (from Gr. 3oXepOs deceptive) to coarse-grained basalts with little or no glass, but it is now usually taken to indicate intrusive rocks, of basic compo sition belonging to the hypabyssal group, and therefore intrusions and not lavas. It is, however, sometimes difficult to ascertain whether a particular sheet is an intrusion or a lava-flow, because in the later stages of volcanicity the lava does not always reach the surface, but may be injected among the earlier flows.
The dolerite group includes both cale-alkaline and alkaline varieties, of which the former are by far the commoner. Their essential minerals are a basic plagioclase felspar (usually ranging from andesine to anorthite, the most typical being labradorite) and augite ; the more basic varieties contain olivine, while in more acid types a little alkali-felspar and quartz may be present, often as micropegmatite (q.v.) . Original hornblende is not very common, though the augite is of ten more or less converted to hornblende by secondary changes. In some varieties enstatite or hypersthene are found, often with quartz. The commonest accessories are ilmenite and apatite. In the alkaline dolerites, often called teschenites, there is rather more alkali-felspar and either nepheline or more commonly analcite.
The dolerites are dark-coloured, heavy rocks, most commonly of even grain, though sometimes carrying phenocrysts of felspar, augite or olivine. The structure is typically ophitic ; i.e., the crystals of felspar of prismatic form are enclosed in large plates of augite, giving a sort of shimmery appearance. The centre of a comparatively large mass may approach a gabbro in texture, while the margins are often of very fine grain, or even glassy, owing to chilling during intrusion by the cold country rock.
Dolerites may form small bosses and laccoliths, thus merging into the gabbros, but the most typical manner of occurrence is as sills and dikes, varying in thickness or width from a mere film to hundreds of feet. Often there is to be observed a considerable variation in composition from the centre to the margins of a large intrusion, owing to differentiation of the magma during cooling, the basic minerals concentrating near the margin, while felspar, including orthoclase or albite and even quartz may be found at the centre. Again it is not infrequent to find an injection of some acid rock in the same dike-fissure, or forming a parallel sheet or sill. This phenomenon is due to differentiation before crystal lization.
The ordinary dolerites, poor in alkalis and correspondingly rich in lime, are undoubtedly the most abundant of all basic in trusives. They occur in great quantity along with surface flows of basalt in innumerable localities. In Britain dolerites, mostly carrying olivine, are abundant in the Tertiary volcanic province of western Scotland and north-eastern Ireland, as well as among the carboniferous eruptives of the central valley of Scotland. Well known examples of enstatite-dolerite (with quartz) are the Great Whin Sill of northern England, and Penmaenmawr in north Wales. By far the greatest known development of dolerite, however, is that of the Karroo region, Natal, etc., in South Africa, where individual sheets may be thousands of feet thick.
Alkaline dolerites (teschenites) are less common; the type example is that of Teschen in Moravia, while a good many occurrences are known in Scotland, of both carboniferous and Tertiary age, and in the English midlands (Clee hills, etc.).
The dolerites are of considerable economic value, as when fresh the stone is hard, heavy and tough, being well-suited for paving setts and road metal.
In America and in mining literature generally the dolerites are often called diabase, but this name is not to be recommended, owing to the different senses in which it has been used in past times. (R. H. RA.)