BOSTONITE

bostonite, in petrology, a fine-grained, pale-coloured, grey or pinkish rock, which consists essentially of alkali-felspar (orthoclase, microperthite, &c.). some of them contain a small amount of interstitial quartz (quartz bostonites); others have a small percentage of lime, which occasions the presence of a plagioclase felspar (maenite, gauteite, lime-bostonite). other minerals, except apatite, zircon and magnetite, are typically absent. they have very much the same composition as the trachytes; and many rocks of this series have been grouped with these or with the orthophyres. typically they occur as dikes or as thin sills, often in association with nepheline-syenite; and they seem to bear a complementary relationship to certain types of lamprophyre, such as camptonite and monchiquite. though nowhere very common they have a wide distribution, being known from scotland, wales, massachusetts, montreal, portugal, bohemia, &c. the lindoites and quartz-lindoites of norway are closely allied to the bostonites.