5. Summary: The actual distribution of igncous rocks as revealed by that of silica in analyses can be shown to consist of two semi-normal curves of error situated at its extreme ends with a U-type distribution in the central part. The distribution can be fitted with two normal curves of error determined by the shape of the end-curves of the actual distribution, revealing a law such that all visible igneous rocks are normally distributed about two types—a basalt and a granite. The divergence from this law in the central region is possibly due in part to errors of sampling, and in part to magmatic mixing. The primary types arose during an early planetary stage. The acid external layer may have arisen under the influence of escaping volatile constituents, which concentrated upwards much silica and alkalis. Neither the syntectic-differentiation theory of Daly nor the one-magma theory of Bowen is capable of accounting for the distribution. The normal suites with their characteristic dispersion can be produced by ‘crystallization-differentiation’ as defined by Bowen.