The Buchanan massif is a Precambrian pluton in the central Texas area, midway between the towns of Llano and Burnet, It shows a roughly concentric zonation over its outcrop area of about; 100 square miles. The general zonation is from fine-grained rocks in the interior of the pluton to coarse-grained and porphyritic rocks near the margin, and the trends of modal composition indicate an increase in acidity toward the margins. This reverse zonation is here explained by an equilibrium (equal fugacity) distribution of water in the intruded magma shortly after emplacement, The higher water content of the cooler, outer zones depresses the crystallization temperature of the magma below the actual temperature in the outer regions, thus allowing the interior to crystallize first. Progressive crystallization outward further increased the water content of the margins, and pegmatitic segregations attest to the saturation of the melt in the outer regions. The high water content maintained crystallization in the two-phase, subsolidus feldspar region throughout most of the pluton. The Enchanted Rock batholith and other less well-known plutons in the central Texas area exhibit the same textural and modal trends as the Buchanan massif, and it is proposed that their crystallization histories are similar. The high water content of the magmas may be indicative of derivation from remelting of crustal rocks.