The Old Red Sandstone lavas of the Sidlaw hills area, eastern Scotland, consist of olivine basalts, andesitic basalts, olivine-free basalts with two pyroxenes, andesite, and trachyandesite, with the basalts approaching andesites. These types are described petrographically, and the relation of augite texture to the order of mineral crystallization is discussed. Comparison is made of the Sidlaw lavas with those of the eastern Forfarshire area, which are quite similar. Chemical analyses are included.

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