The Blue Mountain nepheline syenite sill was formed from a nepheline syenite magma that intruded metasediments and syenite gneiss about 1300 m.y. ago. The magma was fractionated during emplacement into several zones, distinguished by their chemistry, mineralogy, and texture. Pegmatites were formed from late-stage fluids. Mafic bodies in the sill represent slabs of metasediments which were stoped from the roof of the sill by the magma, and which were plasticized and strung out into narrow sheets parallel to the direction of flow of the magma. Strong contortion of some sheets was produced by non-laminar flow of the magma around the included material. A contact aureole was developed above and to a minor extent below the sill; alkalies, alumina, water, and carbon dioxide were added to the metasediments from the magma.The sill and surrounding rocks were folded into their present positions during the Grenville orogeny about 1000 m.y. ago. Country rocks were recrystallized, but the rocks of most of the sill were only slightly affected by the metamorphism accompanying the orogeny.