The Palisades Sill of New Jersey and New York states was emplaced in the Newark Formation in the Upper Triassic, and is about 1000 feet thick. Its form is typical of a hypabyssal sheet except for some dike-like features, which are mainly in the northern part of the intrusion. The sill is a multiple intrusion in which two magma phases of oversaturated tholeiite have been recognized. Olivine crystallized in appreciable amounts only after the emplacement of the second and larger phase. The accumulation of olivine enveloped in early plagioclase and augite determined the location of the famous hyalosiderite dolerite layer above the crystallized part of the first phase. The order in which the essential minerals began to crystallize in each magma phase was olivine, plagioclase, augite, and orthopyroxene. In the northern part of the intrusion, where the Mg-olivine layer is absent, the contact between the two magmas is marked by a reversal in the pyroxene fractionation trend. Equilibrium was established between the two magma phases early in the fractionation sequence. Fractional crystallization dominated the differentiation process, and progressive crystallization proceeded on a normal course to an advanced stage of iron-, silica-, and alkali-enrichment where fayalite granophyre and granophyric dolerite formed. The whole-rock analyses for the major and minor elements, and for B, Nd, Nb, Pb, and Sn, are presented. In addition, element partitioning and the behavior of elements with fractionation have been determined in the sill by studying the variations of Ca, Sr, Ba, Fe, Mn, Ti, Cr, V, Sc, Mg, Co, Ni, Y, Zr, Mo, La, Cu, and Ga in the rock and mineral series. The elements distribute and arrange themselves in crystal lattice sites during fractional crystallization according to their total chemical properties. It is not only the properties of the ion in the liquid, but also its properties within the crystal lattice, which determine its behavior, and the trace cations like the major cations show varying site preference according to their bonding characteristics in different lattice structures.