Although they are contrasting types of acid rocks, the felsites and granophyres of a small area of central Rhum (Scotland), forming the eastern part of the northern edge of the Tertiary igneous complex of the region, are essentially similar chemically and mineralogically, indicating crystallization from the same parent magma. Differences in texture are attributed to crystallization in different environments. Rapid loss of volatiles in the environment of a cauldron subsidence fracture zone occupied by explosion breccia caused chilling of the melt to a glass whose devitrification resulted in formation of the felsites. The granophyres were formed in an environment in which loss of volatiles was less rapid and supercooling was not as great. Both the degree of supercooling and the composition of the magma are critical in the production of the granophyric texture. A composition lying on the cotectic line for the relevant water vapor pressure and a degree of supercooling in the region of the maximum rates of nucleation and crystal growth are prerequisites for the development of the granophyric texture.