The volume discontinuity of cesium at 45,000 kg/${\mathrm{cm}}^{2}$ is attributed to a shift of the valence electrons from the $6s$ zone to the empty $5d$ zones. A Wigner-Seitz calculation for the $6s$ and $5d$ bands supports this explanation. The potential used is described and tabulated. The possible existence of similar transitions for potassium and rubidium is discussed.