The field relations, mineralogy, and petrography of a small Tertiary ultrabasic sheet are briefly described. Chemical analyses show that compositional variations exist within the intrusion and electron-probe analyses demonstrate that optically undetectable zoning is present in the olivine. Throughout the intrusion, which is composed primarily of forsteritic olivine, calcic plagioclase, and clinopyroxene, there is a differential distribution of olivine. This distribution is attributed to flow differentiation during intrusion followed by gravitative sinking of olivine crystals.