LAMPROPHYRES, a group of rocks containing pheno crysts, usually of biotite and hornblende (with bright cleavage surfaces), often also of olivine and augite, but not of felspar. They are thus distinguished from the porphyries and porphyrites in which the felspar has crystallized in two generations. They are essentially "dike rocks," occurring as dikes and thin sills, and are also found as marginal facies of plutonic intrusions. They are named from Gr. Xa,urpos, bright, and the terminal part of the word porphyry (meaning rocks containing bright porphyritic crys tals), and furnish a good example of the correlation which often exists between petrographical types and their mode of occurrence, showing the importance of physical conditions in determining the mineralogical and structural characters of rocks. They are usually dark in colour, owing to the abundance of ferro-magnesian silicates, of relatively high specific gravity and liable to decom position. For these reasons they have been defined as a melano crate series (rich in the dark minerals) ; and they are often ac companied by a complementary leucocratl series (rich in the white minerals felspar and quartz) such as aplites, porphyries and fel sites. Both have been produced by differentiation of a parent magma, and if the two complementary sets of rocks could be mixed in the right proportions a mass of similar chemical compo sition to the parent magma would be produced.
Although the porphyritic structure is almost universal, it is sometimes not very marked. The large biotites and hornblendes are not sharply distinct from those of intermediate size, which in turn graduate into the small crystals of the same minerals in the ground mass. As a rule all the ingredients have rather perfect crystalline forms (except quartz), hence these rocks have been called "panidiomorphic." In many lamprophyres the pale quartz and felspathic ingredients tend to occur in rounded spots, or ocelli, in which there has been progressive crystallization from the margins towards the centre. These spots may consist of radiate or brush-like felspars (with some mica and hornblende) or of quartz and felspar.
There are two great groups of lamprophyres differing in com position while retaining the general features of the class. One
of these accompanies intrusions of granite and diorite and includes the minettes, kersantites, vogesites and spessartites. The other is found in association with nepheline-syenites, essexites and tes chenites, and is exemplified by camptonites, monchiquites and alnoites. The complementary facies of the first group is the aplites, porphyrites and fesites ; that of the second group includes bostonites, tinguaites and other rocks.
The granito-dioritic-lamprophyres (the first of these two groups) are found in many districts where granites and diorites occur, e.g., the Highlands and Southern Uplands of Scotland, the Lake district, Ireland, the Vosges, Black Forest, Harz, etc. As a rule they do not proceed directly from the granite, but form separate dikes which may be later than, and consequently may cut, the granites and diorites. In other districts where granites are abundant no rocks of this class are known. It is rare to find only one member of the group present, but minettes, vogesites, kersantites, etc., all appear and there are usually transitional forms. For this reason these rock species must not be regarded as sharply distinct from one another. The group as a whole is well characterized and shows few transitions to porphyries, porphyrites and other dike types; its subdivisions, however, tend to merge into one another and especially when they are weathered are hard to distinguish. The presence or absence of the four dominant minerals, orthoclase, plagioclase, biotite and hornblende, deter mines the species. Minettes contain biotite and orthoclase ; ker santites, biotite and plagioclase. Vogesites contain hornblende and orthoclase ; spessartites, hornblende and plagioclase. Each variety of lamprophyre may and often does contain all four minerals, but is named according to the two which preponderate ; they con tain also iron oxides (usually titaniferous), apatite, sometimes sphene, augite and olivine, and as all lamprophyres are prone to alteration by weathering a great abundance of secondary minerals is usually found in them, the principal being calcite and other carbonates, limonite, chlorite, quartz and kaolin.