Within the main serpentine of Unst, a lower peridotite layer, a dunite layer, and an upper pyroxenite layer can be distinguished. These three zones have transitional contacts which are marked by a primary mineral banding. Analyses of specimens from each zone are given and it is suggested that the layering is due to differentiation in situ. The zones dip steeply eastward and the base of the intrusion is cut off on the west by a thrust which is slightly oblique to the banding and locally cuts out the peridotite zone. Interior planes of dislocation are parallel to, or perpendicular to, the primary banding.