BOSTONITE, in petrology, a fine-grained, pale-coloured, grey or pinkish rock, which consists essentially of alkali-felspar (orthoclase, anorthoclase, microperthite, etc.). Some bostonites contain a small amount of interstitial quartz (quartz-bostonites) ; others have a small percentage of lime, which occasions the pres ence of a plagioclase felspar (lime-bostonite). Other minerals, except apatite, zircon and magnetite, are typically absent. They have very much the same com position 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 nephe line-syenite.