Abstract. The nodular rocks of the Kragero district range from nodular quartzites, through nodular schists to nodular gneisses. The name nodular 'granite' has, in the past, been applied to the most felspathic members of the series. New analyses show that the composition of the sillimanite quartzite nodules is remarkably constant in that SiO1 and Al2O3 together exceed 95%, while the compositions of the matrices constitute a fairly well-defined chemical group characterized by high SiO2, Al2O3 and a dominance of K2O over Na2O. The matrix composition of the nodular quartzites corresponds to a sandstone and that of the nodular schists to an impure sandstone. The matrix composition of the nodular gneisses ('granites'), with its high normative quartz and K-felspar and negligible plagioclase, resembles many arkoses and differs markedly from normal granite.
The nodular structure is explained by the operation of two phases of oblique folding on a series of rocks with variable competence. The first folding produced rods; the second folding broke the rods into nodules. The two sets of folds have been revealed by recent mapping. It is postulated that the nodular rocks owe their origin to the multiple folding of a series of arkoses and impure sandstones which contained hard ribs of high-alumina quartzite. Neither significant metamorphic differentiation nor felspathization need have taken place.