A suite of highly altered spilitic rocks from an intrusion 100–150 feet thick in the Yeso formation is described. There are many similar intrusions throughout the region. The alteration process is traced in detail. With falling temperatures each metallic element tends to form its own mineral or minerals with Si and Al as the only other cations. Deuteric alteration probably has been accentuated by distillation of hyperfusibles, especially sulphate radicles, from the country rocks which include much gypsum. Fine-to medium-grained albitites are the end products of differentiation, and zircon studies indicate that these rocks are partly of magmatic, partly of metasomatic origin. Five chemical analyses and results of 10 spectrographic analyses are given.