Igneous rocks of the Highwood Mountains, Montana: Part IV. The Stocks

The largest of the stocks of the Highwood Mountains is about 3 miles long. Most of the stocks are made up of shonkinite and mafic syenite. The stock of East Peak is a mafic pseudoleucite-augite syenite. Small bodies of leucite-rich rocks—missourite and niligongite—are present in the Shonkin stock. Leucite malignite makes several small bodies. In the stocks, augite is the predominant mineral, sanidine is the chief felsic mineral, altered nepheline, olivine, and biotite are in moderate amounts, and plagioclase, hornblende, and leucite are rare. The early magmatic sanidine is high in barium. The Highwood Peak stock is made up chiefly of felsic syenite and monzonite.