The Ferrar Sills in the McMurdo Dry Valleys are remarkable in exhibiting a vertical variation in bulk composition from ultramafic in the lowermost, Basement, sill to evolved compositions in the uppermost, Mt. Fleming, sill at the top, overlapping with the compositions of the lavas and volcaniclastics. The system apparently developed from the top down, becoming increasingly capped by overlying materials and the solidification of earlier magma, forcing succeeding magmas to form deeper and more massive sills. And as the system became more mature and the country rock became heated, the later magma came carrying a massive slurry or sludge of large crystals, primocrysts, that furnish as tracers a clear indication how the Basement Sill was filled and how it evolved.