The Silver City Dome contains a rare occurrence of a lamproite sill complex. Interpretation of seismic reflection data suggests that the character of the Late Cretaceous Hills Pond lamproite is a consequence of local stratigraphic conditions—namely, the dense layering and hydrologic impermeability of Pennsylvanian strata that host the intrusion. The structure developed in at least two phases. First, a gentle dome and marginal ring grabens formed as a result of deep magmatic activity. Later, a smaller and steeper dome and a centrally located down-warp were superimposed upon the gentle dome as a result of sill injection into the Pennsylvanian sequence and collapse of the rock column above the magma source.