Nepheline- and analcime-bearing syenite occurs at the upper contact of a quartz diabase sill with the Lockatong Argillite at Brookville, New Jersey. The syenite is intimately mixed with granophyre that formed by fractional crystallization of basaltic magma and accumulated near the top of the sill. Some of the residual liquid now represented by granophyre reacted with silica-poor, sodium-rich argillite to form the syenite. Initial Sr87/Sr86 ratios (assuming an age of 190 m. y.) are 0.7144–0.7147 for the unmetamorphosed and the hornfelsed argillite, 0.7067 for the granophyre, 0.7058 for the diabase, and intermediate values of 0.7128–0.7131 for the syenite. The isotopic evidence agrees with field, petrographic, experimental, and chemical data supporting assimilation. Alternative hypotheses of syenite formation by separate intrusion, differentiation, or partial melting of Lockatong Argillite are contradicted by the data. Isotopic and normative data strongly suggest that syenite formed from argillite and granophyre in the proportions three or four to one; during the assimilation process the country rock acted as solvent and the intrusive as solute.