Summary: The Eskdalemuir tholeiite has the following modal composition: glass 42·2 %, felspar 24·4 %, pyroxene 30·6 %), magnetite 2 %, chlorite 1%. The glass has been separated and analysed. It has 70% SiO₂ and is of granitic composition. On a selected triangular diagram the plots of the Eskdalemuir rock, its residual glass, and its crystallinc portion are spread along a narrow belt marking the compositions of the Tertiary tholeiites. Because the differences in the chemical compositions of the phrases of the Eskdalemuir tholeiite are produced by crystallization, it is concluded that the variety of composition of the other Tertiary tholeiites is consistent with a genesis controlled by crystallization differentiation.