Pleasant Mountain

Pleasant Mountain, which rises to a series of ridges and peaks nearly 500 m above the surrounding rather flat country, is a composite stock intruded into the Chatham granite. The earliest rocks of the stock are large inclusions of volcanics which include trachytic tuffs, breccias and porphyritic trachytes. The oldest intrusive rocks of the stock occur as elongate bodies along the margins, and as remnants within the nordmarkites, which are the last and by far the dominant rocks of the intrusion. The earlier rocks are augite syenites of rather variable texture and composition, some apparently of hybrid origin. On the western edge of the intrusion is an area of analcime syenite, monzonite, and diorite, the last two grading into each other. The analcime syenite is a homogeneous rock consisting of perthite, a little oligoclase, 6-8% analcime, amphibole and accessories. The monzonite contains equal proportions of zoned oligoclase and perthite, biotite and a pale green pyroxene. The diorites are more mafic rocks, lacking alkali feldspar. The final stage was the emplacement of nordmarkite, anorthoclase syenite porphyry and porphyritic syenite. Contacts between these rock types are transitional except for the sharp southwestern contact of the large mass of anorthoclase syenite porphyry in the northeast. The nordmarkite comprises about 80% perthite, 7-15% oligoclase, 3% quartz and a little hornblende and biotite. Dykes of lamprophyre, bostonite, aplite and analcime syenite are found within the intrusion and the surrounding Chatham granite. Chemical analyses are available (Jenks, 1934, Table 4).