The Waipapa Group of Permian-Jurassic age comprises a geosynclinal succession of lithic and feldspathic greywackes and argillites; cherts; and spilitic lavas. Low-grade metamorphism of these rocks has resulted in the development of a mineral assemblage characteristic of the prehnite-pumpellyite metagreywacke facies of Coombs (1960). The major detrital components of the greywackes are feldspar, quartz, lithic fragments, and a matrix. The composition of the greywackes is directly related to their grain size which in turn varies with bed thickness. The largely reconstituted matrix is considered of primary origin in massive, coarse-grained greywackes, but is in part secondary in most medium- and fine-grained rocks. The present study suggests that the conditions most conducive to the formation of a secondary matrix in metagreywackes are the presence of abundant labile detrital particles of fine sand-size, poor sorting, and low resistance of the rocks to shearing stresses. The sedimentary rocks represent a mixture of granitic and acid to intermediate volcanic detritus with a small contribution from a metamorphic source.