The Blue Mountain complex consists of a main mass 4x2.4 km from which extends a southwest-trending arm 6.4 km long and up to 0.4 km wide. It lies in the core of a northeast-southwest-trending syncline of Precambrian metasedimentary rocks including amphibolite, paragneiss and a limestone which lies adjacent to the alkaline complex at many places. Contacts are generally sharp and conformable but the Methuen granite lying to the southeast cuts across the foliated nepheline syenites of Blue Mountain. Payne (1968) suggests that the complex forms a sill, but structural interpretations differ depending on whether the writer favours an igneous or metasomatic origin; for relevant references see Payne (1968) and Duke and Edgar (1977). The complex consists essentially of foliated and folded nepheline syenite gneiss with lesser amounts of syenitic gneiss. A number of zones can be distinguished based on the proportions of biotite and hornblende, or characterized by the presence of muscovite, muscovite-magnetite or the absence or paucity of nepheline. There is some indication of a concentric arrangement with a central core of hornblende- nepheline syenite surrounded by a biotite zone which passes into a muscovite-bearing zone. A pink syenite is concentrated along the border and in the southwestern arm. All the nepheline syenites consist of about 50% albite, 20% microcline and 20-25% nepheline. Also present are biotite, muscovite, iron-rich hastingsite and riebeckite, aegirine-augite, andraditic garnet, magnetite, corundum, calcite, cancrinite and zircon. Sheets of more mafic rocks occur within the nepheline syenites and have been interpreted in a number of ways (Payne, 1968, p. 262), while nepheline-bearing pegmatites are common in some zones of the nepheline syenite. Rock analyses are to be found in Hewitt (1961) and Payne (1968) and mineral analyses in Duke and Edgar (1977). Melting experiments on nepheline syenite in the presence of H2O and CO2 are described by Millhollen (1971).