The pyroxenes of fine grained and coarse grained mafic intrusives and those of mafic extrusives are described. Hypotheses on the trend of crystallization of pyroxenes from basalts are reviewed. The investigation indicates that augite does not grade into pigeonite, but that pigeonite is a definite and distinct mineral variety. Augite and pigeonite may and do crystallize together in equilibrium with each other. Limited, rather than complete, solid solution exists in a portion of the clinopyroxene field. Pigeonite inverts on slow cooling to hypersthene and this inversion may be used as a point on a geological thermometer. Hypersthene inverted from pigeonite can be distinguished from hypersthene of primary crystallization. The temperature of basaltic intrusions does not exceed 1140° C. and norrnally is close to 1120° C.